git-gui: Correct disappearing unstaged files.

A prior commit tried to use the old index state for the old working
directory state during a UI refresh of a file.  This caused files
which were being unstaged (and thus becoming unmodified) to drop
out of the working directory side of the display, at least until
the user performed a rescan to force the UI to redisplay everything.

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

git-gui.sh

@@ -1336,7 +1336,7 @@ proc display_file {path state} {
 	if {[string index $old_m 0] eq {U}} {
 		set o U
 	} else {
-		set o [string index $old_m 0]
+		set o [string index $old_m 1]
 	}
 	if {[string index $new_m 0] eq {U}} {
 		set n U