Fix gitmail's short_refname() to support refs containing slashes
Refs can contain slashes. For example, the short ref for 'refs/heads/foo/bar' is 'foo/bar'. Previously, we just chopped off everything up to and including the last slash character, which doesn't return the correct short ref in this case. Call git-rev-parse to shorten the ref for us. In case it doesn't return anything (which shouldn't happen), fall back to the full ref.