get_default_branch: Use HEAD instead of guessing

[mattcen]

Fixes #307

[mattcen]

Not sure if this is t he nicest way to do things, especially with the decode() and calling head() just so we can catch it if it fails, but it feels less bad than guessing, which is what this replaces. Thoughts and comments welcome.

[jelmer]

Rather than checking the revision that HEAD and other branches point at, you could simply check what HEAD resolves to.

See Repo.refs.follow(b'HEAD')

[mattcen]

@jelmer Thanks; code updated.