Correctly handle refs/patches on series rename

When renaming a series, the refs/patches dir was not moved, and by
chance a new one was created by the repository-upgrade code, but that
left the old one behind as cruft (which the safety checks added in a
former patch now detects).

Also adds a regression test to assert that nothing by the old name
is left behind.

Signed-off-by: Yann Dirson <ydirson@altern.org>