Bump required libgit2 version to 0.21
Remote: make renaming take a method call

Renaming a remote in pygit2 has been done via Remote.name= up to now,
but this is inherently unsafe, as it provides no way to pass up the
refspecs that libgit2 was unable to remap.

In fact, if there ever was such problem, we would have segfaulted.

libgit2 now provides a much more direct way of getting back the results,
so expose it as the return value of Remote.rename(). This also removes
the hint that a rename might be something that happens only to the
in-memory structure.