git-gui: Allow users to run fsck-objects from the gui.

I recently found a need to run fsck-objects in a number of repositories
that I also use git-gui against.  Tossing in a menu option to invoke
fsck-objects and have its output show up in a console window is simple
enough to do.

We probably need to enhance the console window used by fsck-objects,
like to open up the Git fsck-objects manual page and let the user see
what each message means (such as "dangling commit") and to also let the
user invoke prune, to cleanup any such dangling objects.  But right now
I'm going to ignore that problem in favor of getting other more important
features implemented.

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