git-gui: support underline style when parsing diff output

Suggested-by: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>
diff --git a/git-gui.sh b/git-gui.sh
index 21033cb..789c177 100755
--- a/git-gui.sh
+++ b/git-gui.sh
@@ -3376,6 +3376,7 @@
 	$ui_diff tag configure clri3$n -background $c
 }
 $ui_diff tag configure clr1 -font font_diffbold
+$ui_diff tag configure clr4 -underline 1
 
 $ui_diff tag conf d_info -foreground blue -font font_diffbold