remote: "rename o foo" should not rename ref "origin/bar"

remote: "rename o foo" should not rename ref "origin/bar"

When renaming a remote called 'o' using 'git remote rename o foo', git
should also rename any remote-tracking branches for the remote. This
does happen, but any remote-tracking branches starting with
'refs/remotes/o', such as 'refs/remotes/origin/bar', will also be
renamed (to 'refs/remotes/foorigin/bar' in this case).

Fix it by simply matching one more character, up to the slash
following the remote name.

Signed-off-by: Martin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>