Make gowork-snapshot support repositories in detached head

When git repositories are in detached head, git symbolic-ref cannot be used.

In this case, gowork-snapshot outputs fatal: ref origin is not a symbolic ref and create a buildout profile with empty revisions.

After these changes, we try to get from a remote named origin in this case. If this did not work, we output the problematic repository.

I added some minor "cleanups" at the same time.