List-like, so that it's backward compatible. But it has a new method
raise_on_error, that throws an exception if anything failed to push.

Related to #621

delete_head and Head.delete historically accept either Head objects
or a str name of a head.  Adjust the typing to match.  This
unfortunately requires suppressing type warnings in the signature of
RemoteReference.delete, since it inherits from Head but does not
accept str (since it needs access to the richer data of
RemoteReference).

Using assignment to make add an alias for create unfortunately
confuses mypy, since it loses track of the fact that it's a
classmethod and starts treating it like a staticmethod.  Replace
with a stub wrapper instead.