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>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

lib/diff.tcl

@@ -51,11 +51,16 @@ proc force_diff_encoding {enc} {
 
 proc handle_empty_diff {} {
 	global current_diff_path file_states file_lists
+	global diff_empty_count
 
 	set path $current_diff_path
 	set s $file_states($path)
 	if {[lindex $s 0] ne {_M}} return
 
+	# Prevent infinite rescan loops
+	incr diff_empty_count
+	if {$diff_empty_count > 1} return
+
 	info_popup [mc "No differences detected.
 
 %s has no changes.
@@ -310,6 +315,7 @@ proc read_diff {fd cont_info} {
 	global ui_diff diff_active
 	global is_3way_diff is_conflict_diff current_diff_header
 	global current_diff_queue
+	global diff_empty_count
 
 	$ui_diff conf -state normal
 	while {[gets $fd line] >= 0} {
@@ -415,7 +421,10 @@ proc read_diff {fd cont_info} {
 
 		if {[$ui_diff index end] eq {2.0}} {
 			handle_empty_diff
+		} else {
+			set diff_empty_count 0
 		}
+
 		set callback [lindex $cont_info 1]
 		if {$callback ne {}} {
 			eval $callback