Add `set_head_bytes()`

[drmikehenry]

I'd like to have Repository::set_head_bytes() that works like Repository::set_head() but which does not require valid UTF8 (analogous to having both Repository::set_namespace() and Repository::set_namespace_bytes()). Without set_head_bytes(), if a user chooses a branch name that's invalid UTF8, I can't set HEAD to that branch.

Thanks,
Michael Henry