git-gui: Add Refresh to diff viewer context menu.

git-gui: Add Refresh to diff viewer context menu.

Sometimes you want to just force the diff to redisplay itself without
rescanning every file in the filesystem (as that can be very costly
on large projects and slow operating systems).  Now you can force a
diff-only refresh from the context menu.  Previously you could also
do this by reclicking on the file name in the UI, but it may not be
obvious to all users, having a context menu option makes it more
clear.

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