git-gui

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>