git-gui: Added revert changes command.

Users sometimes need to be able to throw away locally modified files
in order to go back to the last committed version of that file.  To
perform a revert the user must first uninclude each file from the new
commit as the working file must at least partially match the index,
and we use git-checkout-index to update the working directory.

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