This lets us look up remotes by name, which is not possible by just
returning the list of remotes.
Move remote creation to Repostiory.remotes.create() and keep the old
Repository.create_remote() for compatibility, delegating to this new
way.
Existing code should keep working, but this moves us towards what we'd
need for a better interface in 0.22 which makes remote renaming and
deleting work with a name rather than an instance and would make sense
to exist as part of an Remote.remotes object.
b80103b017