`gh` is the new `hub`

`gh` is the new `hub`, rewritten to be fast and efficient.
http://owenou.com/gh/

It has been adopted by GitHub themselves and will replace `hub` entirely:
https://github.com/github/hub/issues/475

This patch simply checks for `gh` as a possible `_git_cmd` in addition to 
hub, restoring scm_breeze functionality for people who have switched to gh.