[PATCH (GIT-GUI BUG) v2] git-gui: Avoid an infinite rescan loop in handle_empty_diff.

If the index update machinery and git diff happen to disagree on whether a particular file is modified, it may cause git-gui to enter an infinite index rescan loop, where an empty diff starts a rescan, which finds the same set of files modified, and tries to display the diff for the first one, which happens to be the empty one. A current example of a possible disagreement point is the autocrlf filter.

This patch breaks the loop by using a global counter to track the auto-rescans. The variable is reset whenever a non-empty diff is displayed.

Another suggested approach, which is based on giving the --exit-code argument to git diff, cannot be used, because diff-files seems to trust the timestamps in the index, and returns a non-zero code even if the file is actually unchanged, which essentially defeats the purpose of the auto-rescan logic.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
---

	Apparently git-diff also has a small crlf-related bug.
	It can be reproduced using the following commands:

	$ rm -rf .git *
	$ git init
	$ echo > foo
	$ git add foo && git commit -m init
	$ git diff-files -p --exit-code && echo unchanged
	unchanged
	$ git diff --exit-code && echo unchanged
	unchanged
	$ touch foo
	$ git diff-files -p --exit-code && echo unchanged
	diff --git a/foo b/foo
	$ git diff --exit-code && echo unchanged
	unchanged

	    Without autocrlf git-diff recognizes that
	    the file is unchanged.

	$ rm -rf .git *
	$ git init
	$ git config core.autocrlf true
	$ echo > foo
	$ unix2dos -o foo
	$ git add foo && git commit -m init
	$ git diff-files -p --exit-code && echo unchanged
	unchanged
	$ git diff --exit-code && echo unchanged
	unchanged
	$ touch foo
	$ git diff-files -p --exit-code && echo unchanged
	diff --git a/foo b/foo
	$ git diff --exit-code && echo unchanged
	diff --git a/foo b/foo

	    With crlf it prints an empty diff and says
	    that there are some changes.

	I don't think that this behavior should depend
	on the autocrlf setting.

	-- Alexander

 lib/diff.tcl |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)