git-gui: Allow forced push into remote repository

git-gui: Allow forced push into remote repository

Some workflows allow the user to forcefully update a remote branch,
such as in a "proposed updates" (aka "pu") branch where the branch
is rewound and rebuilt on a daily basis against the current master
branch.  In such a case the "--force" or leading + must be used to
make git-push execute anyway, even though it may be discarding one
or more commits on the remote side.

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