git-gui: Fix diff parsing for lines starting with "--" or "++"

Languages like Lua and SQL use "--" to mark a line as commented out.
If this appears at column 0 and is part of the pre-image we may see
"--- foo" in the diff, indicating that the line whose content is
 "-- foo" has been removed from the new version.

git-gui was incorrectly parsing "--- foo" as the old file name
in the file header, causing it to generate a bad patch file when
the user tried to stage or unstage a hunk or the selected line.
We need to keep track of where we are in the parsing so that we do
not misread a deletion or addition record as part of the header.

Reported-by: Alexander Gladysh <agladysh@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

lib/diff.tcl

@@ -192,6 +192,7 @@ proc show_diff {path w {lno {}} {scroll_pos {}}} {
 		return
 	}
 
+	set ::current_diff_inheader 1
 	fconfigure $fd \
 		-blocking 0 \
 		-encoding binary \
@@ -207,18 +208,21 @@ proc read_diff {fd scroll_pos} {
 	while {[gets $fd line] >= 0} {
 		# -- Cleanup uninteresting diff header lines.
 		#
-		if {   [string match {diff --git *}      $line]
-			|| [string match {diff --cc *}       $line]
-			|| [string match {diff --combined *} $line]
-			|| [string match {--- *}             $line]
-			|| [string match {+++ *}             $line]} {
-			append current_diff_header $line "\n"
-			continue
+		if {$::current_diff_inheader} {
+			if {   [string match {diff --git *}      $line]
+			    || [string match {diff --cc *}       $line]
+			    || [string match {diff --combined *} $line]
+			    || [string match {--- *}             $line]
+			    || [string match {+++ *}             $line]} {
+				append current_diff_header $line "\n"
+				continue
+			}
 		}
 		if {[string match {index *} $line]} continue
 		if {$line eq {deleted file mode 120000}} {
 			set line "deleted symlink"
 		}
+		set ::current_diff_inheader 0
 
 		# -- Automatically detect if this is a 3 way diff.
 		#