15 files changed, 825 insertions, 486 deletions

diff --git a/.gitignore b/.gitignore
index 16f7a97d9..41c0b20a7 100644
--- a/.gitignore
+++ b/.gitignore
@@ -36,6 +36,8 @@ git-diff
 git-diff-files
 git-diff-index
 git-diff-tree
+git-difftool
+git-difftool--helper
 git-describe
 git-fast-export
 git-fast-import
@@ -79,6 +81,7 @@ git-merge-recursive
 git-merge-resolve
 git-merge-subtree
 git-mergetool
+git-mergetool--lib
 git-mktag
 git-mktree
 git-name-rev
diff --git a/Documentation/config.txt b/Documentation/config.txt
index f3ebd2f9a..5319df505 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -667,6 +667,27 @@ diff.suppressBlankEmpty::
 	A boolean to inhibit the standard behavior of printing a space
 	before each empty output line. Defaults to false.
 
+diff.tool::
+	Controls which diff tool is used.  `diff.tool` overrides
+	`merge.tool` when used by linkgit:git-difftool[1] and has
+	the same valid values as `merge.tool` minus "tortoisemerge"
+	and plus "kompare".
+
+difftool.<tool>.path::
+	Override the path for the given tool.  This is useful in case
+	your tool is not in the PATH.
+
+difftool.<tool>.cmd::
+	Specify the command to invoke the specified diff tool.
+	The specified command is evaluated in shell with the following
+	variables available:  'LOCAL' is set to the name of the temporary
+	file containing the contents of the diff pre-image and 'REMOTE'
+	is set to the name of the temporary file containing the contents
+	of the diff post-image.
+
+difftool.prompt::
+	Prompt before each invocation of the diff tool.
+
 diff.wordRegex::
 	A POSIX Extended Regular Expression used to determine what is a "word"
 	when performing word-by-word difference calculations.  Character
diff --git a/contrib/difftool/git-difftool.txt b/Documentation/git-difftool.txt
index 2b7bc03ec..15b247bab 100644
--- a/contrib/difftool/git-difftool.txt
+++ b/Documentation/git-difftool.txt
@@ -3,35 +3,37 @@ git-difftool(1)
 
 NAME
 ----
-git-difftool - compare changes using common merge tools
+git-difftool - Show changes using common diff tools
 
 SYNOPSIS
 --------
-'git difftool' [--tool=<tool>] [--no-prompt] ['git diff' options]
+'git difftool' [--tool=<tool>] [-y|--no-prompt|--prompt] [<'git diff' options>]
 
 DESCRIPTION
 -----------
 'git-difftool' is a git command that allows you to compare and edit files
-between revisions using common merge tools.  At its most basic level,
-'git-difftool' does what 'git-mergetool' does but its use is for non-merge
-situations such as when preparing commits or comparing changes against
-the index.
-
-'git difftool' is a frontend to 'git diff' and accepts the same
-arguments and options.
-
-See linkgit:git-diff[1] for the full list of supported options.
+between revisions using common diff tools.  'git difftool' is a frontend
+to 'git-diff' and accepts the same options and arguments.
 
 OPTIONS
 -------
+-y::
+--no-prompt::
+	Do not prompt before launching a diff tool.
+
+--prompt::
+	Prompt before each invocation of the diff tool.
+	This is the default behaviour; the option is provided to
+	override any configuration settings.
+
 -t <tool>::
 --tool=<tool>::
-	Use the merge resolution program specified by <tool>.
+	Use the diff tool specified by <tool>.
 	Valid merge tools are:
-	kdiff3, kompare, tkdiff, meld, xxdiff, emerge,
-	vimdiff, gvimdiff, ecmerge, and opendiff
+	kdiff3, kompare, tkdiff, meld, xxdiff, emerge, vimdiff, gvimdiff,
+	ecmerge, diffuse and opendiff
 +
-If a merge resolution program is not specified, 'git-difftool'
+If a diff tool is not specified, 'git-difftool'
 will use the configuration variable `diff.tool`.  If the
 configuration variable `diff.tool` is not set, 'git-difftool'
 will pick a suitable default.
@@ -42,7 +44,7 @@ can configure the absolute path to kdiff3 by setting
 `difftool.kdiff3.path`. Otherwise, 'git-difftool' assumes the
 tool is available in PATH.
 +
-Instead of running one of the known merge tool programs,
+Instead of running one of the known diff tools,
 'git-difftool' can be customized to run an alternative program
 by specifying the command line to invoke in a configuration
 variable `difftool.<tool>.cmd`.
@@ -56,8 +58,7 @@ is set to the name of the temporary file containing the contents
 of the diff post-image.  `$BASE` is provided for compatibility
 with custom merge tool commands and has the same value as `$LOCAL`.
 
---no-prompt::
-	Do not prompt before launching a diff tool.
+See linkgit:git-diff[1] for the full list of supported options.
 
 CONFIG VARIABLES
 ----------------
@@ -65,20 +66,19 @@ CONFIG VARIABLES
 difftool equivalents have not been defined.
 
 diff.tool::
-	The default merge tool to use.
+	The default diff tool to use.
 
 difftool.<tool>.path::
 	Override the path for the given tool.  This is useful in case
 	your tool is not in the PATH.
 
 difftool.<tool>.cmd::
-	Specify the command to invoke the specified merge tool.
+	Specify the command to invoke the specified diff tool.
 +
 See the `--tool=<tool>` option above for more details.
 
-merge.keepBackup::
-	The original, unedited file content can be saved to a file with
-	a `.orig` extension.  Defaults to `true` (i.e. keep the backup files).
+difftool.prompt::
+	Prompt before each invocation of the diff tool.
 
 SEE ALSO
 --------
diff --git a/Documentation/git-mergetool--lib.txt b/Documentation/git-mergetool--lib.txt
new file mode 100644
index 000000000..78eb03f0a
--- /dev/null
+++ b/Documentation/git-mergetool--lib.txt
@@ -0,0 +1,54 @@
+git-mergetool--lib(1)
+=====================
+
+NAME
+----
+git-mergetool--lib - Common git merge tool shell scriptlets
+
+SYNOPSIS
+--------
+'TOOL_MODE=(diff|merge) . "$(git --exec-path)/git-mergetool--lib"'
+
+DESCRIPTION
+-----------
+
+This is not a command the end user would want to run.  Ever.
+This documentation is meant for people who are studying the
+Porcelain-ish scripts and/or are writing new ones.
+
+The 'git-mergetool--lib' scriptlet is designed to be sourced (using
+`.`) by other shell scripts to set up functions for working
+with git merge tools.
+
+Before sourcing 'git-mergetool--lib', your script must set `TOOL_MODE`
+to define the operation mode for the functions listed below.
+'diff' and 'merge' are valid values.
+
+FUNCTIONS
+---------
+get_merge_tool::
+	returns a merge tool.
+
+get_merge_tool_cmd::
+	returns the custom command for a merge tool.
+
+get_merge_tool_path::
+	returns the custom path for a merge tool.
+
+run_merge_tool::
+	launches a merge tool given the tool name and a true/false
+	flag to indicate whether a merge base is present.
+	'$MERGED', '$LOCAL', '$REMOTE', and '$BASE' must be defined
+	for use by the merge tool.
+
+Author
+------
+Written by David Aguilar <davvid@gmail.com>
+
+Documentation
+--------------
+Documentation by David Aguilar and the git-list <git@vger.kernel.org>.
+
+GIT
+---
+Part of the linkgit:git[1] suite
diff --git a/Documentation/git-mergetool.txt b/Documentation/git-mergetool.txt
index 5d3c63287..ff9700d17 100644
--- a/Documentation/git-mergetool.txt
+++ b/Documentation/git-mergetool.txt
@@ -26,7 +26,8 @@ OPTIONS
 --tool=<tool>::
 	Use the merge resolution program specified by <tool>.
 	Valid merge tools are:
-	kdiff3, tkdiff, meld, xxdiff, emerge, vimdiff, gvimdiff, ecmerge, and opendiff
+	kdiff3, tkdiff, meld, xxdiff, emerge, vimdiff, gvimdiff, ecmerge,
+	diffuse, tortoisemerge and opendiff
 +
 If a merge resolution program is not specified, 'git-mergetool'
 will use the configuration variable `merge.tool`.  If the
diff --git a/Documentation/merge-config.txt b/Documentation/merge-config.txt
index 1ff08ff2c..4832bc75e 100644
--- a/Documentation/merge-config.txt
+++ b/Documentation/merge-config.txt
@@ -22,7 +22,8 @@ merge.stat::
 merge.tool::
 	Controls which merge resolution program is used by
 	linkgit:git-mergetool[1].  Valid built-in values are: "kdiff3",
-	"tkdiff", "meld", "xxdiff", "emerge", "vimdiff", "gvimdiff", and
+	"tkdiff", "meld", "xxdiff", "emerge", "vimdiff", "gvimdiff",
+	"diffuse", "ecmerge", "tortoisemerge", and
 	"opendiff".  Any other value is treated is custom merge tool
 	and there must be a corresponding mergetool.<tool>.cmd option.
 
diff --git a/Makefile b/Makefile
index 7c8763710..57f9f2625 100644
--- a/Makefile
+++ b/Makefile
@@ -279,12 +279,14 @@ TEST_PROGRAMS =
 
 SCRIPT_SH += git-am.sh
 SCRIPT_SH += git-bisect.sh
+SCRIPT_SH += git-difftool--helper.sh
 SCRIPT_SH += git-filter-branch.sh
 SCRIPT_SH += git-lost-found.sh
 SCRIPT_SH += git-merge-octopus.sh
 SCRIPT_SH += git-merge-one-file.sh
 SCRIPT_SH += git-merge-resolve.sh
 SCRIPT_SH += git-mergetool.sh
+SCRIPT_SH += git-mergetool--lib.sh
 SCRIPT_SH += git-parse-remote.sh
 SCRIPT_SH += git-pull.sh
 SCRIPT_SH += git-quiltimport.sh
@@ -298,6 +300,7 @@ SCRIPT_SH += git-submodule.sh
 SCRIPT_SH += git-web--browse.sh
 
 SCRIPT_PERL += git-add--interactive.perl
+SCRIPT_PERL += git-difftool.perl
 SCRIPT_PERL += git-archimport.perl
 SCRIPT_PERL += git-cvsexportcommit.perl
 SCRIPT_PERL += git-cvsimport.perl
diff --git a/command-list.txt b/command-list.txt
index 3583a33ee..fb03a2ebb 100644
--- a/command-list.txt
+++ b/command-list.txt
@@ -33,6 +33,7 @@ git-diff                                mainporcelain common
 git-diff-files                          plumbinginterrogators
 git-diff-index                          plumbinginterrogators
 git-diff-tree                           plumbinginterrogators
+git-difftool                            ancillaryinterrogators
 git-fast-export				ancillarymanipulators
 git-fast-import				ancillarymanipulators
 git-fetch                               mainporcelain common
diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
index c1c879821..1a90cb87f 100755
--- a/contrib/completion/git-completion.bash
+++ b/contrib/completion/git-completion.bash
@@ -910,6 +910,26 @@ _git_diff ()
 	__git_complete_file
 }
 
+__git_mergetools_common="diffuse ecmerge emerge kdiff3 meld opendiff
+			tkdiff vimdiff gvimdiff xxdiff
+"
+
+_git_difftool ()
+{
+	local cur="${COMP_WORDS[COMP_CWORD]}"
+	case "$cur" in
+	--tool=*)
+		__gitcomp "$__git_mergetools_common kompare" "" "${cur##--tool=}"
+		return
+		;;
+	--*)
+		__gitcomp "--tool="
+		return
+		;;
+	esac
+	COMPREPLY=()
+}
+
 __git_fetch_options="
 	--quiet --verbose --append --upload-pack --force --keep --depth=
 	--tags --no-tags
@@ -1172,10 +1192,7 @@ _git_mergetool ()
 	local cur="${COMP_WORDS[COMP_CWORD]}"
 	case "$cur" in
 	--tool=*)
-		__gitcomp "
-			kdiff3 tkdiff meld xxdiff emerge
-			vimdiff gvimdiff ecmerge opendiff
-			" "" "${cur##--tool=}"
+		__gitcomp "$__git_mergetools_common tortoisemerge" "" "${cur##--tool=}"
 		return
 		;;
 	--*)
@@ -1901,6 +1918,7 @@ _git ()
 	config)      _git_config ;;
 	describe)    _git_describe ;;
 	diff)        _git_diff ;;
+	difftool)    _git_difftool ;;
 	fetch)       _git_fetch ;;
 	format-patch) _git_format_patch ;;
 	fsck)        _git_fsck ;;
diff --git a/contrib/difftool/git-difftool-helper b/contrib/difftool/git-difftool-helper
deleted file mode 100755
index 9c0a13452..000000000
--- a/contrib/difftool/git-difftool-helper
+++ /dev/null
@@ -1,249 +0,0 @@
-#!/bin/sh
-# git-difftool-helper is a GIT_EXTERNAL_DIFF-compatible diff tool launcher.
-# It supports kdiff3, kompare, tkdiff, xxdiff, meld, opendiff,
-# emerge, ecmerge, vimdiff, gvimdiff, and custom user-configurable tools.
-# This script is typically launched by using the 'git difftool'
-# convenience command.
-#
-# Copyright (c) 2009 David Aguilar
-
-# Set GIT_DIFFTOOL_NO_PROMPT to bypass the per-file prompt.
-should_prompt () {
-	! test -n "$GIT_DIFFTOOL_NO_PROMPT"
-}
-
-# Should we keep the backup .orig file?
-keep_backup_mode="$(git config --bool merge.keepBackup || echo true)"
-keep_backup () {
-	test "$keep_backup_mode" = "true"
-}
-
-# This function manages the backup .orig file.
-# A backup $MERGED.orig file is created if changes are detected.
-cleanup_temp_files () {
-	if test -n "$MERGED"; then
-		if keep_backup && test "$MERGED" -nt "$BACKUP"; then
-			test -f "$BACKUP" && mv -- "$BACKUP" "$MERGED.orig"
-		else
-			rm -f -- "$BACKUP"
-		fi
-	fi
-}
-
-# This is called when users Ctrl-C out of git-difftool-helper
-sigint_handler () {
-	cleanup_temp_files
-	exit 1
-}
-
-# This function prepares temporary files and launches the appropriate
-# merge tool.
-launch_merge_tool () {
-	# Merged is the filename as it appears in the work tree
-	# Local is the contents of a/filename
-	# Remote is the contents of b/filename
-	# Custom merge tool commands might use $BASE so we provide it
-	MERGED="$1"
-	LOCAL="$2"
-	REMOTE="$3"
-	BASE="$1"
-	ext="$$$(expr "$MERGED" : '.*\(\.[^/]*\)$')"
-	BACKUP="$MERGED.BACKUP.$ext"
-
-	# Create and ensure that we clean up $BACKUP
-	test -f "$MERGED" && cp -- "$MERGED" "$BACKUP"
-	trap sigint_handler INT
-
-	# $LOCAL and $REMOTE are temporary files so prompt
-	# the user with the real $MERGED name before launching $merge_tool.
-	if should_prompt; then
-		printf "\nViewing: '$MERGED'\n"
-		printf "Hit return to launch '%s': " "$merge_tool"
-		read ans
-	fi
-
-	# Run the appropriate merge tool command
-	case "$merge_tool" in
-	kdiff3)
-		basename=$(basename "$MERGED")
-		"$merge_tool_path" --auto \
-			--L1 "$basename (A)" \
-			--L2 "$basename (B)" \
-			-o "$MERGED" "$LOCAL" "$REMOTE" \
-			> /dev/null 2>&1
-		;;
-
-	kompare)
-		"$merge_tool_path" "$LOCAL" "$REMOTE"
-		;;
-
-	tkdiff)
-		"$merge_tool_path" -o "$MERGED" "$LOCAL" "$REMOTE"
-		;;
-
-	meld)
-		"$merge_tool_path" "$LOCAL" "$REMOTE"
-		;;
-
-	vimdiff)
-		"$merge_tool_path" -c "wincmd l" "$LOCAL" "$REMOTE"
-		;;
-
-	gvimdiff)
-		"$merge_tool_path" -c "wincmd l" -f "$LOCAL" "$REMOTE"
-		;;
-
-	xxdiff)
-		"$merge_tool_path" \
-			-X \
-			-R 'Accel.SaveAsMerged: "Ctrl-S"' \
-			-R 'Accel.Search: "Ctrl+F"' \
-			-R 'Accel.SearchForward: "Ctrl-G"' \
-			--merged-file "$MERGED" \
-			"$LOCAL" "$REMOTE"
-		;;
-
-	opendiff)
-		"$merge_tool_path" "$LOCAL" "$REMOTE" \
-			-merge "$MERGED" | cat
-		;;
-
-	ecmerge)
-		"$merge_tool_path" "$LOCAL" "$REMOTE" \
-			--default --mode=merge2 --to="$MERGED"
-		;;
-
-	emerge)
-		"$merge_tool_path" -f emerge-files-command \
-			"$LOCAL" "$REMOTE" "$(basename "$MERGED")"
-		;;
-
-	*)
-		if test -n "$merge_tool_cmd"; then
-			( eval $merge_tool_cmd )
-		fi
-		;;
-	esac
-
-	cleanup_temp_files
-}
-
-# Verifies that (difftool|mergetool).<tool>.cmd exists
-valid_custom_tool() {
-	merge_tool_cmd="$(git config difftool.$1.cmd)"
-	test -z "$merge_tool_cmd" &&
-	merge_tool_cmd="$(git config mergetool.$1.cmd)"
-	test -n "$merge_tool_cmd"
-}
-
-# Verifies that the chosen merge tool is properly setup.
-# Built-in merge tools are always valid.
-valid_tool() {
-	case "$1" in
-	kdiff3 | kompare | tkdiff | xxdiff | meld | opendiff | emerge | vimdiff | gvimdiff | ecmerge)
-		;; # happy
-	*)
-		if ! valid_custom_tool "$1"
-		then
-			return 1
-		fi
-		;;
-	esac
-}
-
-# Sets up the merge_tool_path variable.
-# This handles the difftool.<tool>.path configuration.
-# This also falls back to mergetool defaults.
-init_merge_tool_path() {
-	merge_tool_path=$(git config difftool."$1".path)
-	test -z "$merge_tool_path" &&
-	merge_tool_path=$(git config mergetool."$1".path)
-	if test -z "$merge_tool_path"; then
-		case "$1" in
-		emerge)
-			merge_tool_path=emacs
-			;;
-		*)
-			merge_tool_path="$1"
-			;;
-		esac
-	fi
-}
-
-# Allow GIT_DIFF_TOOL and GIT_MERGE_TOOL to provide default values
-test -n "$GIT_MERGE_TOOL" && merge_tool="$GIT_MERGE_TOOL"
-test -n "$GIT_DIFF_TOOL" && merge_tool="$GIT_DIFF_TOOL"
-
-# If merge tool was not specified then use the diff.tool
-# configuration variable.  If that's invalid then reset merge_tool.
-# Fallback to merge.tool.
-if test -z "$merge_tool"; then
-	merge_tool=$(git config diff.tool)
-	test -z "$merge_tool" &&
-	merge_tool=$(git config merge.tool)
-	if test -n "$merge_tool" && ! valid_tool "$merge_tool"; then
-		echo >&2 "git config option diff.tool set to unknown tool: $merge_tool"
-		echo >&2 "Resetting to default..."
-		unset merge_tool
-	fi
-fi
-
-# Try to guess an appropriate merge tool if no tool has been set.
-if test -z "$merge_tool"; then
-	# We have a $DISPLAY so try some common UNIX merge tools
-	if test -n "$DISPLAY"; then
-		# If gnome then prefer meld, otherwise, prefer kdiff3 or kompare
-		if test -n "$GNOME_DESKTOP_SESSION_ID" ; then
-			merge_tool_candidates="meld kdiff3 kompare tkdiff xxdiff gvimdiff"
-		else
-			merge_tool_candidates="kdiff3 kompare tkdiff xxdiff meld gvimdiff"
-		fi
-	fi
-	if echo "${VISUAL:-$EDITOR}" | grep 'emacs' > /dev/null 2>&1; then
-		# $EDITOR is emacs so add emerge as a candidate
-		merge_tool_candidates="$merge_tool_candidates emerge opendiff vimdiff"
-	elif echo "${VISUAL:-$EDITOR}" | grep 'vim' > /dev/null 2>&1; then
-		# $EDITOR is vim so add vimdiff as a candidate
-		merge_tool_candidates="$merge_tool_candidates vimdiff opendiff emerge"
-	else
-		merge_tool_candidates="$merge_tool_candidates opendiff emerge vimdiff"
-	fi
-	echo "merge tool candidates: $merge_tool_candidates"
-
-	# Loop over each candidate and stop when a valid merge tool is found.
-	for i in $merge_tool_candidates
-	do
-		init_merge_tool_path $i
-		if type "$merge_tool_path" > /dev/null 2>&1; then
-			merge_tool=$i
-			break
-		fi
-	done
-
-	if test -z "$merge_tool" ; then
-		echo "No known merge resolution program available."
-		exit 1
-	fi
-
-else
-	# A merge tool has been set, so verify that it's valid.
-	if ! valid_tool "$merge_tool"; then
-		echo >&2 "Unknown merge tool $merge_tool"
-		exit 1
-	fi
-
-	init_merge_tool_path "$merge_tool"
-
-	if test -z "$merge_tool_cmd" && ! type "$merge_tool_path" > /dev/null 2>&1; then
-		echo "The merge tool $merge_tool is not available as '$merge_tool_path'"
-		exit 1
-	fi
-fi
-
-
-# Launch the merge tool on each path provided by 'git diff'
-while test $# -gt 6
-do
-	launch_merge_tool "$1" "$2" "$5"
-	shift 7
-done
diff --git a/git-difftool--helper.sh b/git-difftool--helper.sh
new file mode 100755
index 000000000..57e8e3256
--- /dev/null
+++ b/git-difftool--helper.sh
@@ -0,0 +1,59 @@
+#!/bin/sh
+# git-difftool--helper is a GIT_EXTERNAL_DIFF-compatible diff tool launcher.
+# This script is typically launched by using the 'git difftool'
+# convenience command.
+#
+# Copyright (c) 2009 David Aguilar
+
+# Load common functions from git-mergetool--lib
+TOOL_MODE=diff
+. git-mergetool--lib
+
+# difftool.prompt controls the default prompt/no-prompt behavior
+# and is overridden with $GIT_DIFFTOOL*_PROMPT.
+should_prompt () {
+	prompt=$(git config --bool difftool.prompt || echo true)
+	if test "$prompt" = true; then
+		test -z "$GIT_DIFFTOOL_NO_PROMPT"
+	else
+		test -n "$GIT_DIFFTOOL_PROMPT"
+	fi
+}
+
+# Sets up shell variables and runs a merge tool
+launch_merge_tool () {
+	# Merged is the filename as it appears in the work tree
+	# Local is the contents of a/filename
+	# Remote is the contents of b/filename
+	# Custom merge tool commands might use $BASE so we provide it
+	MERGED="$1"
+	LOCAL="$2"
+	REMOTE="$3"
+	BASE="$1"
+
+	# $LOCAL and $REMOTE are temporary files so prompt
+	# the user with the real $MERGED name before launching $merge_tool.
+	if should_prompt; then
+		printf "\nViewing: '$MERGED'\n"
+		printf "Hit return to launch '%s': " "$merge_tool"
+		read ans
+	fi
+
+	# Run the appropriate merge tool command
+	run_merge_tool "$merge_tool"
+}
+
+# Allow GIT_DIFF_TOOL and GIT_MERGE_TOOL to provide default values
+test -n "$GIT_MERGE_TOOL" && merge_tool="$GIT_MERGE_TOOL"
+test -n "$GIT_DIFF_TOOL" && merge_tool="$GIT_DIFF_TOOL"
+
+if test -z "$merge_tool"; then
+	merge_tool="$(get_merge_tool)" || exit
+fi
+
+# Launch the merge tool on each path provided by 'git diff'
+while test $# -gt 6
+do
+	launch_merge_tool "$1" "$2" "$5"
+	shift 7
+done
diff --git a/contrib/difftool/git-difftool b/git-difftool.perl
index 0deda3a0e..948ff7f6f 100755
--- a/contrib/difftool/git-difftool
+++ b/git-difftool.perl
@@ -2,9 +2,12 @@
 # Copyright (c) 2009 David Aguilar
 #
 # This is a wrapper around the GIT_EXTERNAL_DIFF-compatible
-# git-difftool-helper script.  This script exports
-# GIT_EXTERNAL_DIFF and GIT_PAGER for use by git, and
-# GIT_DIFFTOOL_NO_PROMPT and GIT_DIFF_TOOL for use by git-difftool-helper.
+# git-difftool--helper script.
+#
+# This script exports GIT_EXTERNAL_DIFF and GIT_PAGER for use by git.
+# GIT_DIFFTOOL_NO_PROMPT, GIT_DIFFTOOL_PROMPT, and GIT_DIFF_TOOL
+# are exported for use by git-difftool--helper.
+#
 # Any arguments that are unknown to this script are forwarded to 'git diff'.
 
 use strict;
@@ -18,7 +21,7 @@ my $DIR = abs_path(dirname($0));
 sub usage
 {
 	print << 'USAGE';
-usage: git difftool [--tool=<tool>] [--no-prompt] ["git diff" options]
+usage: git difftool [--tool=<tool>] [-y|--no-prompt] ["git diff" options]
 USAGE
 	exit 1;
 }
@@ -27,13 +30,16 @@ sub setup_environment
 {
 	$ENV{PATH} = "$DIR:$ENV{PATH}";
 	$ENV{GIT_PAGER} = '';
-	$ENV{GIT_EXTERNAL_DIFF} = 'git-difftool-helper';
+	$ENV{GIT_EXTERNAL_DIFF} = 'git-difftool--helper';
 }
 
 sub exe
 {
 	my $exe = shift;
-	return defined $ENV{COMSPEC} ? "$exe.exe" : $exe;
+	if ($^O eq 'MSWin32' || $^O eq 'msys') {
+		return "$exe.exe";
+	}
+	return $exe;
 }
 
 sub generate_command
@@ -47,7 +53,7 @@ sub generate_command
 			$skip_next = 0;
 			next;
 		}
-		if ($arg eq '-t' or $arg eq '--tool') {
+		if ($arg eq '-t' || $arg eq '--tool') {
 			usage() if $#ARGV <= $idx;
 			$ENV{GIT_DIFF_TOOL} = $ARGV[$idx + 1];
 			$skip_next = 1;
@@ -57,11 +63,17 @@ sub generate_command
 			$ENV{GIT_DIFF_TOOL} = substr($arg, 7);
 			next;
 		}
-		if ($arg eq '--no-prompt') {
+		if ($arg eq '-y' || $arg eq '--no-prompt') {
 			$ENV{GIT_DIFFTOOL_NO_PROMPT} = 'true';
+			delete $ENV{GIT_DIFFTOOL_PROMPT};
+			next;
+		}
+		if ($arg eq '--prompt') {
+			$ENV{GIT_DIFFTOOL_PROMPT} = 'true';
+			delete $ENV{GIT_DIFFTOOL_NO_PROMPT};
 			next;
 		}
-		if ($arg eq '-h' or $arg eq '--help') {
+		if ($arg eq '-h' || $arg eq '--help') {
 			usage();
 		}
 		push @command, $arg;
diff --git a/git-mergetool--lib.sh b/git-mergetool--lib.sh
new file mode 100644
index 000000000..a16a2795d
--- /dev/null
+++ b/git-mergetool--lib.sh
@@ -0,0 +1,385 @@
+# git-mergetool--lib is a library for common merge tool functions
+diff_mode() {
+	test "$TOOL_MODE" = diff
+}
+
+merge_mode() {
+	test "$TOOL_MODE" = merge
+}
+
+translate_merge_tool_path () {
+	case "$1" in
+	vimdiff)
+		echo vim
+		;;
+	gvimdiff)
+		echo gvim
+		;;
+	emerge)
+		echo emacs
+		;;
+	*)
+		echo "$1"
+		;;
+	esac
+}
+
+check_unchanged () {
+	if test "$MERGED" -nt "$BACKUP"; then
+		status=0
+	else
+		while true; do
+			echo "$MERGED seems unchanged."
+			printf "Was the merge successful? [y/n] "
+			read answer < /dev/tty
+			case "$answer" in
+			y*|Y*) status=0; break ;;
+			n*|N*) status=1; break ;;
+			esac
+		done
+	fi
+}
+
+valid_tool () {
+	case "$1" in
+	kdiff3 | tkdiff | xxdiff | meld | opendiff | \
+	emerge | vimdiff | gvimdiff | ecmerge | diffuse)
+		;; # happy
+	tortoisemerge)
+		if ! merge_mode; then
+			return 1
+		fi
+		;;
+	kompare)
+		if ! diff_mode; then
+			return 1
+		fi
+		;;
+	*)
+		if test -z "$(get_merge_tool_cmd "$1")"; then
+			return 1
+		fi
+		;;
+	esac
+}
+
+get_merge_tool_cmd () {
+	# Prints the custom command for a merge tool
+	if test -n "$1"; then
+		merge_tool="$1"
+	else
+		merge_tool="$(get_merge_tool)"
+	fi
+	if diff_mode; then
+		echo "$(git config difftool.$merge_tool.cmd ||
+		        git config mergetool.$merge_tool.cmd)"
+	else
+		echo "$(git config mergetool.$merge_tool.cmd)"
+	fi
+}
+
+run_merge_tool () {
+	merge_tool_path="$(get_merge_tool_path "$1")" || exit
+	base_present="$2"
+	status=0
+
+	case "$1" in
+	kdiff3)
+		if merge_mode; then
+			if $base_present; then
+				("$merge_tool_path" --auto \
+					--L1 "$MERGED (Base)" \
+					--L2 "$MERGED (Local)" \
+					--L3 "$MERGED (Remote)" \
+					-o "$MERGED" \
+					"$BASE" "$LOCAL" "$REMOTE" \
+				> /dev/null 2>&1)
+			else
+				("$merge_tool_path" --auto \
+					--L1 "$MERGED (Local)" \
+					--L2 "$MERGED (Remote)" \
+					-o "$MERGED" \
+					"$LOCAL" "$REMOTE" \
+				> /dev/null 2>&1)
+			fi
+			status=$?
+		else
+			("$merge_tool_path" --auto \
+				--L1 "$MERGED (A)" \
+				--L2 "$MERGED (B)" "$LOCAL" "$REMOTE" \
+			> /dev/null 2>&1)
+		fi
+		;;
+	kompare)
+		"$merge_tool_path" "$LOCAL" "$REMOTE"
+		;;
+	tkdiff)
+		if merge_mode; then
+			if $base_present; then
+				"$merge_tool_path" -a "$BASE" \
+					-o "$MERGED" "$LOCAL" "$REMOTE"
+			else
+				"$merge_tool_path" \
+					-o "$MERGED" "$LOCAL" "$REMOTE"
+			fi
+			status=$?
+		else
+			"$merge_tool_path" "$LOCAL" "$REMOTE"
+		fi
+		;;
+	meld)
+		if merge_mode; then
+			touch "$BACKUP"
+			"$merge_tool_path" "$LOCAL" "$MERGED" "$REMOTE"
+			check_unchanged
+		else
+			"$merge_tool_path" "$LOCAL" "$REMOTE"
+		fi
+		;;
+	diffuse)
+		if merge_mode; then
+			touch "$BACKUP"
+			if $base_present; then
+				"$merge_tool_path" \
+					"$LOCAL" "$MERGED" "$REMOTE" \
+					"$BASE" | cat
+			else
+				"$merge_tool_path" \
+					"$LOCAL" "$MERGED" "$REMOTE" | cat
+			fi
+			check_unchanged
+		else
+			"$merge_tool_path" "$LOCAL" "$REMOTE" | cat
+		fi
+		;;
+	vimdiff)
+		if merge_mode; then
+			touch "$BACKUP"
+			"$merge_tool_path" -d -c "wincmd l" \
+				"$LOCAL" "$MERGED" "$REMOTE"
+			check_unchanged
+		else
+			"$merge_tool_path" -d -c "wincmd l" \
+				"$LOCAL" "$REMOTE"
+		fi
+		;;
+	gvimdiff)
+		if merge_mode; then
+			touch "$BACKUP"
+			"$merge_tool_path" -d -c "wincmd l" -f \
+				"$LOCAL" "$MERGED" "$REMOTE"
+			check_unchanged
+		else
+			"$merge_tool_path" -d -c "wincmd l" -f \
+				"$LOCAL" "$REMOTE"
+		fi
+		;;
+	xxdiff)
+		if merge_mode; then
+			touch "$BACKUP"
+			if $base_present; then
+				"$merge_tool_path" -X --show-merged-pane \
+					-R 'Accel.SaveAsMerged: "Ctrl-S"' \
+					-R 'Accel.Search: "Ctrl+F"' \
+					-R 'Accel.SearchForward: "Ctrl-G"' \
+					--merged-file "$MERGED" \
+					"$LOCAL" "$BASE" "$REMOTE"
+			else
+				"$merge_tool_path" -X $extra \
+					-R 'Accel.SaveAsMerged: "Ctrl-S"' \
+					-R 'Accel.Search: "Ctrl+F"' \
+					-R 'Accel.SearchForward: "Ctrl-G"' \
+					--merged-file "$MERGED" \
+					"$LOCAL" "$REMOTE"
+			fi
+			check_unchanged
+		else
+			"$merge_tool_path" \
+				-R 'Accel.Search: "Ctrl+F"' \
+				-R 'Accel.SearchForward: "Ctrl-G"' \
+				"$LOCAL" "$REMOTE"
+		fi
+		;;
+	opendiff)
+		if merge_mode; then
+			touch "$BACKUP"
+			if $base_present; then
+				"$merge_tool_path" "$LOCAL" "$REMOTE" \
+					-ancestor "$BASE" \
+					-merge "$MERGED" | cat
+			else
+				"$merge_tool_path" "$LOCAL" "$REMOTE" \
+					-merge "$MERGED" | cat
+			fi
+			check_unchanged
+		else
+			"$merge_tool_path" "$LOCAL" "$REMOTE" | cat
+		fi
+		;;
+	ecmerge)
+		if merge_mode; then
+			touch "$BACKUP"
+			if $base_present; then
+				"$merge_tool_path" "$BASE" "$LOCAL" "$REMOTE" \
+					--default --mode=merge3 --to="$MERGED"
+			else
+				"$merge_tool_path" "$LOCAL" "$REMOTE" \
+					--default --mode=merge2 --to="$MERGED"
+			fi
+			check_unchanged
+		else
+			"$merge_tool_path" "$LOCAL" "$REMOTE" \
+				--default --mode=merge2 --to="$MERGED"
+		fi
+		;;
+	emerge)
+		if merge_mode; then
+			if $base_present; then
+				"$merge_tool_path" \
+					-f emerge-files-with-ancestor-command \
+					"$LOCAL" "$REMOTE" "$BASE" \
+					"$(basename "$MERGED")"
+			else
+				"$merge_tool_path" \
+					-f emerge-files-command \
+					"$LOCAL" "$REMOTE" \
+					"$(basename "$MERGED")"
+			fi
+			status=$?
+		else
+			"$merge_tool_path" -f emerge-files-command \
+				"$LOCAL" "$REMOTE" "$(basename "$MERGED")"
+		fi
+		;;
+	tortoisemerge)
+		if $base_present; then
+			touch "$BACKUP"
+			"$merge_tool_path" \
+				-base:"$BASE" -mine:"$LOCAL" \
+				-theirs:"$REMOTE" -merged:"$MERGED"
+			check_unchanged
+		else
+			echo "TortoiseMerge cannot be used without a base" 1>&2
+			status=1
+		fi
+		;;
+	*)
+		merge_tool_cmd="$(get_merge_tool_cmd "$1")"
+		if test -z "$merge_tool_cmd"; then
+			if merge_mode; then
+				status=1
+			fi
+			break
+		fi
+		if merge_mode; then
+			trust_exit_code="$(git config --bool \
+				mergetool."$1".trustExitCode || echo false)"
+			if test "$trust_exit_code" = "false"; then
+				touch "$BACKUP"
+				( eval $merge_tool_cmd )
+				check_unchanged
+			else
+				( eval $merge_tool_cmd )
+				status=$?
+			fi
+		else
+			( eval $merge_tool_cmd )
+		fi
+		;;
+	esac
+	return $status
+}
+
+guess_merge_tool () {
+	if merge_mode; then
+		tools="tortoisemerge"
+	else
+		tools="kompare"
+	fi
+	if test -n "$DISPLAY"; then
+		if test -n "$GNOME_DESKTOP_SESSION_ID" ; then
+			tools="meld opendiff kdiff3 tkdiff xxdiff $tools"
+		else
+			tools="opendiff kdiff3 tkdiff xxdiff meld $tools"
+		fi
+		tools="$tools gvimdiff diffuse ecmerge"
+	fi
+	if echo "${VISUAL:-$EDITOR}" | grep emacs > /dev/null 2>&1; then
+		# $EDITOR is emacs so add emerge as a candidate
+		tools="$tools emerge vimdiff"
+	elif echo "${VISUAL:-$EDITOR}" | grep vim > /dev/null 2>&1; then
+		# $EDITOR is vim so add vimdiff as a candidate
+		tools="$tools vimdiff emerge"
+	else
+		tools="$tools emerge vimdiff"
+	fi
+	echo >&2 "merge tool candidates: $tools"
+
+	# Loop over each candidate and stop when a valid merge tool is found.
+	for i in $tools
+	do
+		merge_tool_path="$(translate_merge_tool_path "$i")"
+		if type "$merge_tool_path" > /dev/null 2>&1; then
+			echo "$i"
+			return 0
+		fi
+	done
+
+	echo >&2 "No known merge resolution program available."
+	return 1
+}
+
+get_configured_merge_tool () {
+	# Diff mode first tries diff.tool and falls back to merge.tool.
+	# Merge mode only checks merge.tool
+	if diff_mode; then
+		merge_tool=$(git config diff.tool || git config merge.tool)
+	else
+		merge_tool=$(git config merge.tool)
+	fi
+	if test -n "$merge_tool" && ! valid_tool "$merge_tool"; then
+		echo >&2 "git config option $TOOL_MODE.tool set to unknown tool: $merge_tool"
+		echo >&2 "Resetting to default..."
+		return 1
+	fi
+	echo "$merge_tool"
+}
+
+get_merge_tool_path () {
+	# A merge tool has been set, so verify that it's valid.
+	if test -n "$1"; then
+		merge_tool="$1"
+	else
+		merge_tool="$(get_merge_tool)"
+	fi
+	if ! valid_tool "$merge_tool"; then
+		echo >&2 "Unknown merge tool $merge_tool"
+		exit 1
+	fi
+	if diff_mode; then
+		merge_tool_path=$(git config difftool."$merge_tool".path ||
+		                  git config mergetool."$merge_tool".path)
+	else
+		merge_tool_path=$(git config mergetool."$merge_tool".path)
+	fi
+	if test -z "$merge_tool_path"; then
+		merge_tool_path="$(translate_merge_tool_path "$merge_tool")"
+	fi
+	if test -z "$(get_merge_tool_cmd "$merge_tool")" &&
+	! type "$merge_tool_path" > /dev/null 2>&1; then
+		echo >&2 "The $TOOL_MODE tool $merge_tool is not available as"\
+		         "'$merge_tool_path'"
+		exit 1
+	fi
+	echo "$merge_tool_path"
+}
+
+get_merge_tool () {
+	# Check if a merge tool has been configured
+	merge_tool=$(get_configured_merge_tool)
+	# Try to guess an appropriate merge tool if no tool has been set.
+	if test -z "$merge_tool"; then
+		merge_tool="$(guess_merge_tool)" || exit
+	fi
+	echo "$merge_tool"
+}
diff --git a/git-mergetool.sh b/git-mergetool.sh
index 87fa88af5..b52a7410b 100755
--- a/git-mergetool.sh
+++ b/git-mergetool.sh
@@ -11,7 +11,9 @@
 USAGE='[--tool=tool] [-y|--no-prompt|--prompt] [file to merge] ...'
 SUBDIRECTORY_OK=Yes
 OPTIONS_SPEC=
+TOOL_MODE=merge
 . git-sh-setup
+. git-mergetool--lib
 require_work_tree
 
 # Returns true if the mode reflects a symlink
@@ -110,22 +112,6 @@ resolve_deleted_merge () {
 	done
 }
 
-check_unchanged () {
-    if test "$MERGED" -nt "$BACKUP" ; then
-	status=0;
-    else
-	while true; do
-	    echo "$MERGED seems unchanged."
-	    printf "Was the merge successful? [y/n] "
-	    read answer < /dev/tty
-	    case "$answer" in
-		y*|Y*) status=0; break ;;
-		n*|N*) status=1; break ;;
-	    esac
-	done
-    fi
-}
-
 checkout_staged_file () {
     tmpfile=$(expr "$(git checkout-index --temp --stage="$1" "$2")" : '\([^	]*\)	')
 
@@ -137,7 +123,7 @@ checkout_staged_file () {
 merge_file () {
     MERGED="$1"
 
-    f=`git ls-files -u -- "$MERGED"`
+    f=$(git ls-files -u -- "$MERGED")
     if test -z "$f" ; then
 	if test ! -f "$MERGED" ; then
 	    echo "$MERGED: file not found"
@@ -156,9 +142,9 @@ merge_file () {
     mv -- "$MERGED" "$BACKUP"
     cp -- "$BACKUP" "$MERGED"
 
-    base_mode=`git ls-files -u -- "$MERGED" | awk '{if ($3==1) print $1;}'`
-    local_mode=`git ls-files -u -- "$MERGED" | awk '{if ($3==2) print $1;}'`
-    remote_mode=`git ls-files -u -- "$MERGED" | awk '{if ($3==3) print $1;}'`
+    base_mode=$(git ls-files -u -- "$MERGED" | awk '{if ($3==1) print $1;}')
+    local_mode=$(git ls-files -u -- "$MERGED" | awk '{if ($3==2) print $1;}')
+    remote_mode=$(git ls-files -u -- "$MERGED" | awk '{if ($3==3) print $1;}')
 
     base_present   && checkout_staged_file 1 "$MERGED" "$BASE"
     local_present  && checkout_staged_file 2 "$MERGED" "$LOCAL"
@@ -188,97 +174,13 @@ merge_file () {
 	read ans
     fi
 
-    case "$merge_tool" in
-	kdiff3)
-	    if base_present ; then
-		("$merge_tool_path" --auto --L1 "$MERGED (Base)" --L2 "$MERGED (Local)" --L3 "$MERGED (Remote)" \
-		    -o "$MERGED" "$BASE" "$LOCAL" "$REMOTE" > /dev/null 2>&1)
-	    else
-		("$merge_tool_path" --auto --L1 "$MERGED (Local)" --L2 "$MERGED (Remote)" \
-		    -o "$MERGED" "$LOCAL" "$REMOTE" > /dev/null 2>&1)
-	    fi
-	    status=$?
-	    ;;
-	tkdiff)
-	    if base_present ; then
-		"$merge_tool_path" -a "$BASE" -o "$MERGED" "$LOCAL" "$REMOTE"
-	    else
-		"$merge_tool_path" -o "$MERGED" "$LOCAL" "$REMOTE"
-	    fi
-	    status=$?
-	    ;;
-	meld)
-	    touch "$BACKUP"
-	    "$merge_tool_path" "$LOCAL" "$MERGED" "$REMOTE"
-	    check_unchanged
-	    ;;
-	vimdiff)
-	    touch "$BACKUP"
-	    "$merge_tool_path" -c "wincmd l" "$LOCAL" "$MERGED" "$REMOTE"
-	    check_unchanged
-	    ;;
-	gvimdiff)
-	    touch "$BACKUP"
-	    "$merge_tool_path" -c "wincmd l" -f "$LOCAL" "$MERGED" "$REMOTE"
-	    check_unchanged
-	    ;;
-	xxdiff)
-	    touch "$BACKUP"
-	    if base_present ; then
-		"$merge_tool_path" -X --show-merged-pane \
-		    -R 'Accel.SaveAsMerged: "Ctrl-S"' \
-		    -R 'Accel.Search: "Ctrl+F"' \
-		    -R 'Accel.SearchForward: "Ctrl-G"' \
-		    --merged-file "$MERGED" "$LOCAL" "$BASE" "$REMOTE"
-	    else
-		"$merge_tool_path" -X --show-merged-pane \
-		    -R 'Accel.SaveAsMerged: "Ctrl-S"' \
-		    -R 'Accel.Search: "Ctrl+F"' \
-		    -R 'Accel.SearchForward: "Ctrl-G"' \
-		    --merged-file "$MERGED" "$LOCAL" "$REMOTE"
-	    fi
-	    check_unchanged
-	    ;;
-	opendiff)
-	    touch "$BACKUP"
-	    if base_present; then
-		"$merge_tool_path" "$LOCAL" "$REMOTE" -ancestor "$BASE" -merge "$MERGED" | cat
-	    else
-		"$merge_tool_path" "$LOCAL" "$REMOTE" -merge "$MERGED" | cat
-	    fi
-	    check_unchanged
-	    ;;
-	ecmerge)
-	    touch "$BACKUP"
-	    if base_present; then
-		"$merge_tool_path" "$BASE" "$LOCAL" "$REMOTE" --default --mode=merge3 --to="$MERGED"
-	    else
-		"$merge_tool_path" "$LOCAL" "$REMOTE" --default --mode=merge2 --to="$MERGED"
-	    fi
-	    check_unchanged
-	    ;;
-	emerge)
-	    if base_present ; then
-		"$merge_tool_path" -f emerge-files-with-ancestor-command "$LOCAL" "$REMOTE" "$BASE" "$(basename "$MERGED")"
-	    else
-		"$merge_tool_path" -f emerge-files-command "$LOCAL" "$REMOTE" "$(basename "$MERGED")"
-	    fi
-	    status=$?
-	    ;;
-	*)
-	    if test -n "$merge_tool_cmd"; then
-		if test "$merge_tool_trust_exit_code" = "false"; then
-		    touch "$BACKUP"
-		    ( eval $merge_tool_cmd )
-		    check_unchanged
-		else
-		    ( eval $merge_tool_cmd )
-		    status=$?
-		fi
-	    fi
-	    ;;
-    esac
-    if test "$status" -ne 0; then
+    if base_present; then
+	    present=true
+    else
+	    present=false
+    fi
+
+    if ! run_merge_tool "$merge_tool" "$present"; then
 	echo "merge of $MERGED failed" 1>&2
 	mv -- "$BACKUP" "$MERGED"
 
@@ -308,7 +210,7 @@ do
 	-t|--tool*)
 	    case "$#,$1" in
 		*,*=*)
-		    merge_tool=`expr "z$1" : 'z-[^=]*=\(.*\)'`
+		    merge_tool=$(expr "z$1" : 'z-[^=]*=\(.*\)')
 		    ;;
 		1,*)
 		    usage ;;
@@ -337,38 +239,6 @@ do
     shift
 done
 
-valid_custom_tool()
-{
-    merge_tool_cmd="$(git config mergetool.$1.cmd)"
-    test -n "$merge_tool_cmd"
-}
-
-valid_tool() {
-	case "$1" in
-		kdiff3 | tkdiff | xxdiff | meld | opendiff | emerge | vimdiff | gvimdiff | ecmerge)
-			;; # happy
-		*)
-			if ! valid_custom_tool "$1"; then
-				return 1
-			fi
-			;;
-	esac
-}
-
-init_merge_tool_path() {
-	merge_tool_path=`git config mergetool.$1.path`
-	if test -z "$merge_tool_path" ; then
-		case "$1" in
-			emerge)
-				merge_tool_path=emacs
-				;;
-			*)
-				merge_tool_path=$1
-				;;
-		esac
-	fi
-}
-
 prompt_after_failed_merge() {
     while true; do
 	printf "Continue merging other unresolved paths (y/n) ? "
@@ -387,67 +257,16 @@ prompt_after_failed_merge() {
 }
 
 if test -z "$merge_tool"; then
-    merge_tool=`git config merge.tool`
-    if test -n "$merge_tool" && ! valid_tool "$merge_tool"; then
-	    echo >&2 "git config option merge.tool set to unknown tool: $merge_tool"
-	    echo >&2 "Resetting to default..."
-	    unset merge_tool
-    fi
-fi
-
-if test -z "$merge_tool" ; then
-    if test -n "$DISPLAY"; then
-        if test -n "$GNOME_DESKTOP_SESSION_ID" ; then
-            merge_tool_candidates="meld kdiff3 tkdiff xxdiff gvimdiff"
-        else
-            merge_tool_candidates="kdiff3 tkdiff xxdiff meld gvimdiff"
-        fi
-    fi
-    if echo "${VISUAL:-$EDITOR}" | grep 'emacs' > /dev/null 2>&1; then
-        merge_tool_candidates="$merge_tool_candidates emerge opendiff vimdiff"
-    elif echo "${VISUAL:-$EDITOR}" | grep 'vim' > /dev/null 2>&1; then
-        merge_tool_candidates="$merge_tool_candidates vimdiff opendiff emerge"
-    else
-        merge_tool_candidates="$merge_tool_candidates opendiff emerge vimdiff"
-    fi
-    echo "merge tool candidates: $merge_tool_candidates"
-    for i in $merge_tool_candidates; do
-        init_merge_tool_path $i
-        if type "$merge_tool_path" > /dev/null 2>&1; then
-            merge_tool=$i
-            break
-        fi
-    done
-    if test -z "$merge_tool" ; then
-	echo "No known merge resolution program available."
-	exit 1
-    fi
-else
-    if ! valid_tool "$merge_tool"; then
-        echo >&2 "Unknown merge_tool $merge_tool"
-        exit 1
-    fi
-
-    init_merge_tool_path "$merge_tool"
-
-    merge_keep_backup="$(git config --bool merge.keepBackup || echo true)"
-    merge_keep_temporaries="$(git config --bool mergetool.keepTemporaries || echo false)"
-
-    if test -z "$merge_tool_cmd" && ! type "$merge_tool_path" > /dev/null 2>&1; then
-        echo "The merge tool $merge_tool is not available as '$merge_tool_path'"
-        exit 1
-    fi
-
-    if ! test -z "$merge_tool_cmd"; then
-        merge_tool_trust_exit_code="$(git config --bool mergetool.$merge_tool.trustExitCode || echo false)"
-    fi
+    merge_tool=$(get_merge_tool "$merge_tool") || exit
 fi
+merge_keep_backup="$(git config --bool mergetool.keepBackup || echo true)"
+merge_keep_temporaries="$(git config --bool mergetool.keepTemporaries || echo false)"
 
 last_status=0
 rollup_status=0
 
 if test $# -eq 0 ; then
-    files=`git ls-files -u | sed -e 's/^[^	]*	//' | sort -u`
+    files=$(git ls-files -u | sed -e 's/^[^	]*	//' | sort -u)
     if test -z "$files" ; then
 	echo "No files need merging"
 	exit 0
diff --git a/t/t7800-difftool.sh b/t/t7800-difftool.sh
new file mode 100755
index 000000000..2586f864a
--- /dev/null
+++ b/t/t7800-difftool.sh
@@ -0,0 +1,211 @@
+#!/bin/sh
+#
+# Copyright (c) 2009 David Aguilar
+#
+
+test_description='git-difftool
+
+Testing basic diff tool invocation
+'
+
+. ./test-lib.sh
+
+remove_config_vars()
+{
+	# Unset all config variables used by git-difftool
+	git config --unset diff.tool
+	git config --unset difftool.test-tool.cmd
+	git config --unset difftool.prompt
+	git config --unset merge.tool
+	git config --unset mergetool.test-tool.cmd
+	return 0
+}
+
+restore_test_defaults()
+{
+	# Restores the test defaults used by several tests
+	remove_config_vars
+	unset GIT_DIFF_TOOL
+	unset GIT_MERGE_TOOL
+	unset GIT_DIFFTOOL_PROMPT
+	unset GIT_DIFFTOOL_NO_PROMPT
+	git config diff.tool test-tool &&
+	git config difftool.test-tool.cmd 'cat $LOCAL'
+}
+
+prompt_given()
+{
+	prompt="$1"
+	test "$prompt" = "Hit return to launch 'test-tool': branch"
+}
+
+# Create a file on master and change it on branch
+test_expect_success 'setup' '
+	echo master >file &&
+	git add file &&
+	git commit -m "added file" &&
+
+	git checkout -b branch master &&
+	echo branch >file &&
+	git commit -a -m "branch changed file" &&
+	git checkout master
+'
+
+# Configure a custom difftool.<tool>.cmd and use it
+test_expect_success 'custom commands' '
+	restore_test_defaults &&
+	git config difftool.test-tool.cmd "cat \$REMOTE" &&
+
+	diff=$(git difftool --no-prompt branch) &&
+	test "$diff" = "master" &&
+
+	restore_test_defaults &&
+	diff=$(git difftool --no-prompt branch) &&
+	test "$diff" = "branch"
+'
+
+# Ensures that git-difftool ignores bogus --tool values
+test_expect_success 'difftool ignores bad --tool values' '
+	diff=$(git difftool --no-prompt --tool=bogus-tool branch)
+	test "$?" = 1 &&
+	test "$diff" = ""
+'
+
+# Specify the diff tool using $GIT_DIFF_TOOL
+test_expect_success 'GIT_DIFF_TOOL variable' '
+	git config --unset diff.tool
+	GIT_DIFF_TOOL=test-tool &&
+	export GIT_DIFF_TOOL &&
+
+	diff=$(git difftool --no-prompt branch) &&
+	test "$diff" = "branch" &&
+
+	restore_test_defaults
+'
+
+# Test the $GIT_*_TOOL variables and ensure
+# that $GIT_DIFF_TOOL always wins unless --tool is specified
+test_expect_success 'GIT_DIFF_TOOL overrides' '
+	git config diff.tool bogus-tool &&
+	git config merge.tool bogus-tool &&
+
+	GIT_MERGE_TOOL=test-tool &&
+	export GIT_MERGE_TOOL &&
+	diff=$(git difftool --no-prompt branch) &&
+	test "$diff" = "branch" &&
+	unset GIT_MERGE_TOOL &&
+
+	GIT_MERGE_TOOL=bogus-tool &&
+	GIT_DIFF_TOOL=test-tool &&
+	export GIT_MERGE_TOOL &&
+	export GIT_DIFF_TOOL &&
+
+	diff=$(git difftool --no-prompt branch) &&
+	test "$diff" = "branch" &&
+
+	GIT_DIFF_TOOL=bogus-tool &&
+	export GIT_DIFF_TOOL &&
+
+	diff=$(git difftool --no-prompt --tool=test-tool branch) &&
+	test "$diff" = "branch" &&
+
+	restore_test_defaults
+'
+
+# Test that we don't have to pass --no-prompt to difftool
+# when $GIT_DIFFTOOL_NO_PROMPT is true
+test_expect_success 'GIT_DIFFTOOL_NO_PROMPT variable' '
+	GIT_DIFFTOOL_NO_PROMPT=true &&
+	export GIT_DIFFTOOL_NO_PROMPT &&
+
+	diff=$(git difftool branch) &&
+	test "$diff" = "branch" &&
+
+	restore_test_defaults
+'
+
+# git-difftool supports the difftool.prompt variable.
+# Test that GIT_DIFFTOOL_PROMPT can override difftool.prompt = false
+test_expect_success 'GIT_DIFFTOOL_PROMPT variable' '
+	git config difftool.prompt false &&
+	GIT_DIFFTOOL_PROMPT=true &&
+	export GIT_DIFFTOOL_PROMPT &&
+
+	prompt=$(echo | git difftool --prompt branch | tail -1) &&
+	prompt_given "$prompt" &&
+
+	restore_test_defaults
+'
+
+# Test that we don't have to pass --no-prompt when difftool.prompt is false
+test_expect_success 'difftool.prompt config variable is false' '
+	git config difftool.prompt false &&
+
+	diff=$(git difftool branch) &&
+	test "$diff" = "branch" &&
+
+	restore_test_defaults
+'
+
+# Test that the -y flag can override difftool.prompt = true
+test_expect_success 'difftool.prompt can overridden with -y' '
+	git config difftool.prompt true &&
+
+	diff=$(git difftool -y branch) &&
+	test "$diff" = "branch" &&
+
+	restore_test_defaults
+'
+
+# Test that the --prompt flag can override difftool.prompt = false
+test_expect_success 'difftool.prompt can overridden with --prompt' '
+	git config difftool.prompt false &&
+
+	prompt=$(echo | git difftool --prompt branch | tail -1) &&
+	prompt_given "$prompt" &&
+
+	restore_test_defaults
+'
+
+# Test that the last flag passed on the command-line wins
+test_expect_success 'difftool last flag wins' '
+	diff=$(git difftool --prompt --no-prompt branch) &&
+	test "$diff" = "branch" &&
+
+	restore_test_defaults &&
+
+	prompt=$(echo | git difftool --no-prompt --prompt branch | tail -1) &&
+	prompt_given "$prompt" &&
+
+	restore_test_defaults
+'
+
+# git-difftool falls back to git-mergetool config variables
+# so test that behavior here
+test_expect_success 'difftool + mergetool config variables' '
+	remove_config_vars
+	git config merge.tool test-tool &&
+	git config mergetool.test-tool.cmd "cat \$LOCAL" &&
+
+	diff=$(git difftool --no-prompt branch) &&
+	test "$diff" = "branch" &&
+
+	# set merge.tool to something bogus, diff.tool to test-tool
+	git config merge.tool bogus-tool &&
+	git config diff.tool test-tool &&
+
+	diff=$(git difftool --no-prompt branch) &&
+	test "$diff" = "branch" &&
+
+	restore_test_defaults
+'
+
+test_expect_success 'difftool.<tool>.path' '
+	git config difftool.tkdiff.path echo &&
+	diff=$(git difftool --tool=tkdiff --no-prompt branch) &&
+	git config --unset difftool.tkdiff.path &&
+	lines=$(echo "$diff" | grep file | wc -l) &&
+	test "$lines" -eq 1
+'
+
+test_done