Fix Git failures when master-side Gerrit step used

The Git step fetches from the wrong branch when being notified by
Gerrit 'ref-updated' events. In that case, branch=None is passed to
Git.startVC(...), hence the resulting git-fetch call tries to fetch HEAD
instead of the desired branch.

This was likely a typo from the original author.
Merge branch 'fix-gerrit-step' of git://github.com/krf/buildbot