git-gui: Use catch rather than array names to check file.

When we reshow the current diff file it can be faster to just fetch
the value from the file_states array than it is to ask for all paths
whose name exactly matches the one we want to show.  This is because
[array names -exact] is O(n) in the number of files.
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

git-gui

@@ -350,11 +350,11 @@ proc clear_diff {} {
 proc reshow_diff {} {
 	global ui_fname_value ui_status_value file_states
 
-	if {$ui_fname_value != {} && [array names file_states \
-		-exact $ui_fname_value] != {}}  {
-		show_diff $ui_fname_value
-	} else {
+	if {$ui_fname_value == {}
+		|| [catch {set s $file_states($ui_fname_value)}]} {
 		clear_diff
+	} else {
+		show_diff $ui_fname_value
 	}
 }