Teach bash how to complete +refspec on git-push

Teach bash how to complete +refspec on git-push

Using `git push origin +foo` to forcefully overwrite the remote
branch named foo is a common idiom, especially since + is shorter
than the long option --force and can be specified on a per-branch
basis.

We now complete `git push origin +foo` just like we do the standard
`git push origin foo`.  The leading + on a branch refspec does not
alter the completion.

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