Show original and resulting blob object info in diff output.

This adds more cruft to diff --git header to record the blob SHA1 and
the mode the patch/diff is intended to be applied against, to help the
receiving end fall back on a three-way merge.  The new header looks
like this:

    diff --git a/apply.c b/apply.c
    index 7be5041..8366082 100644
    --- a/apply.c
    +++ b/apply.c
    @@ -14,6 +14,7 @@
     //    files that are being modified, but doesn't apply the patch
     //  --stat does just a diffstat, and doesn't actually apply
    +//  --show-index-info shows the old and new index info for...
    ...

Upon receiving such a patch, if the patch did not apply cleanly to the
target tree, the recipient can try to find the matching old objects in
her object database and create a temporary tree, apply the patch to
that temporary tree, and attempt a 3-way merge between the patched
temporary tree and the target tree using the original temporary tree
as the common ancestor.

The patch lifts the code to compute the hash for an on-filesystem
object from update-index.c and makes it available to the diff output
routine.

Signed-off-by: Junio C Hamano <junkio@cox.net>

4 files changed, 14 insertions, 5 deletions

diff --git a/t/diff-lib.sh b/t/diff-lib.sh
index a912f435a..745a1b031 100755
--- a/t/diff-lib.sh
+++ b/t/diff-lib.sh
@@ -29,7 +29,13 @@ compare_diff_raw_z () {
 compare_diff_patch () {
     # When heuristics are improved, the score numbers would change.
     # Ignore them while comparing.
-    sed -e '/^[dis]*imilarity index [0-9]*%$/d' <"$1" >.tmp-1
-    sed -e '/^[dis]*imilarity index [0-9]*%$/d' <"$2" >.tmp-2
+    sed -e '
+	/^[dis]*imilarity index [0-9]*%$/d
+	/^index [0-9a-f]*\.\.[0-9a-f]/d
+    ' <"$1" >.tmp-1
+    sed -e '
+	/^[dis]*imilarity index [0-9]*%$/d
+	/^index [0-9a-f]*\.\.[0-9a-f]/d
+    ' <"$2" >.tmp-2
     diff -u .tmp-1 .tmp-2 && rm -f .tmp-1 .tmp-2
 }
diff --git a/t/t4000-diff-format.sh b/t/t4000-diff-format.sh
index f3b6330a9..beb6d8f48 100755
--- a/t/t4000-diff-format.sh
+++ b/t/t4000-diff-format.sh
@@ -7,6 +7,7 @@ test_description='Test built-in diff output engine.
 
 '
 . ./test-lib.sh
+. ../diff-lib.sh
 
 echo >path0 'Line 1
 Line 2
@@ -48,6 +49,6 @@ EOF
 
 test_expect_success \
     'validate git-diff-files -p output.' \
-    'cmp -s current expected'
+    'compare_diff_patch current expected'
 
 test_done
diff --git a/t/t4001-diff-rename.sh b/t/t4001-diff-rename.sh
index be4748568..2e3c20d6b 100755
--- a/t/t4001-diff-rename.sh
+++ b/t/t4001-diff-rename.sh
@@ -7,6 +7,7 @@ test_description='Test rename detection in diff engine.
 
 '
 . ./test-lib.sh
+. ../diff-lib.sh
 
 echo >path0 'Line 1
 Line 2
@@ -61,6 +62,6 @@ EOF
 
 test_expect_success \
     'validate the output.' \
-    'diff -I "similarity.*" >/dev/null current expected'
+    'compare_diff_patch current expected'
 
 test_done
diff --git a/t/t4004-diff-rename-symlink.sh b/t/t4004-diff-rename-symlink.sh
index f59614ae2..a23aaa0a9 100755
--- a/t/t4004-diff-rename-symlink.sh
+++ b/t/t4004-diff-rename-symlink.sh
@@ -10,6 +10,7 @@ copy of symbolic links, but should not produce rename/copy followed
 by an edit for them.
 '
 . ./test-lib.sh
+. ../diff-lib.sh
 
 test_expect_success \
     'prepare reference tree' \
@@ -61,6 +62,6 @@ EOF
 
 test_expect_success \
     'validate diff output' \
-    'diff -u current expected'
+    'compare_diff_patch current expected'
 
 test_done