remotes can be renamed without tinkering with .git/refs/hg/remote-name, closes #14

Although .git/refs/hg/old-remote-name is not renamed or deleted
automatically, it is not harmful and the user can delete it at any time,
thus

    git remote rename old-remote-name new-remote-name

behaves acceptably with no further user interaction. Update
test_pull_named_remote to reflect this.