Projects/Gitolite: Make the default branch the real default.

Gitlab will now tell gitolite to update the HEAD pointer over ssh using
the symbolic-ref command - this ensures that when performing a git
clone, the branch you receive if none is specified will actually be the
one you configured as default.

This also resolves breakage when your repo lacks any branch named
"master.

Depends on gitlabhq/gitolite#4