New --dirstat=lines mode, doing dirstat analysis based on diffstat

This patch adds an alternative implementation of show_dirstat(), called
show_dirstat_by_line(), which uses the more expensive diffstat analysis
(as opposed to show_dirstat()'s own (relatively inexpensive) analysis)
to derive the numbers from which the --dirstat output is computed.

The alternative implementation is controlled by the new "lines" parameter
to the --dirstat option (or the diff.dirstat config variable).

For binary files, the diffstat analysis counts bytes instead of lines,
so to prevent binary files from dominating the dirstat results, the
byte counts for binary files are divided by 64 before being compared to
their textual/line-based counterparts. This is a stupid and ugly - but
very cheap - heuristic.

In linux-2.6.git, running the three different --dirstat modes:

  time git diff v2.6.20..v2.6.30 --dirstat=changes > /dev/null
vs.
  time git diff v2.6.20..v2.6.30 --dirstat=lines > /dev/null
vs.
  time git diff v2.6.20..v2.6.30 --dirstat=files > /dev/null

yields the following average runtimes on my machine:

 - "changes" (default): ~6.0 s
 - "lines":             ~9.6 s
 - "files":             ~0.1 s

So, as expected, there's a considerable performance hit (~60%) by going
through the full diffstat analysis as compared to the default "changes"
analysis (obviously, "files" is much faster than both). As such, the
"lines" mode is probably only useful if you really need the --dirstat
numbers to be consistent with the numbers returned from the other
--*stat options.

The patch also includes documentation and tests for the new dirstat mode.

Improved-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/Documentation/diff-config.txt b/Documentation/diff-config.txt
index 228329d4a..1aed79e7d 100644
--- a/Documentation/diff-config.txt
+++ b/Documentation/diff-config.txt
@@ -23,6 +23,14 @@ diff.dirstat::
 	the amount of pure code movements within a file.  In other words,
 	rearranging lines in a file is not counted as much as other changes.
 	This is the default behavior when no parameter is given.
+`lines`;;
+	Compute the dirstat numbers by doing the regular line-based diff
+	analysis, and summing the removed/added line counts. (For binary
+	files, count 64-byte chunks instead, since binary files have no
+	natural concept of lines). This is a more expensive `--dirstat`
+	behavior than the `changes` behavior, but it does count rearranged
+	lines within a file as much as other changes. The resulting output
+	is consistent with what you get from the other `--*stat` options.
 `files`;;
 	Compute the dirstat numbers by counting the number of files changed.
 	Each changed file counts equally in the dirstat analysis. This is