git-gui: Handle file mode changes (644->755) in diff viewer

Johannes Sixt pointed out the diff headers "old mode ..." and
"new mode ..." were not being parsed properly by git-gui.  We
now include them in the diff viewer for a file.

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

lib/diff.tcl

@@ -220,6 +220,7 @@ proc read_diff {fd} {
 
 		if {[string match {mode *} $line]
 			|| [string match {new file *} $line]
+			|| [regexp {^(old|new) mode *} $line]
 			|| [string match {deleted file *} $line]
 			|| [string match {deleted symlink} $line]
 			|| [string match {Binary files * and * differ} $line]