git-gui: Added arbitrary branch pushing support.

git-gui: Added arbitrary branch pushing support.

Because its common for some users to push topic branches to a remote
repository for review and merging by other parties, users need an
easy way to push one or more branches to a remote repository without
needing to edit their .git/config file anytime their set of active
branches changes.

We now provide a basic 'Push...' menu action in the Push menu which
opens a dialog allowing the user to select from their set of local
branches (refs/heads, minus tracking branches).  The user can designate
which repository to send the changes to by selecting from an already
configured remote or by entering any valid Git URL.

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