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

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: Shawn O. Pearce <spearce@spearce.org>