Projects/Gitlab-shell: Make the default branch the real default.

Gitlab will now tell gitlab-shell to update the HEAD pointer using the
update-head 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 commit 0cb0f8c in gitlab-shell