remote: only update remote-tracking branch if updating refspec

remote: only update remote-tracking branch if updating refspec

'git remote rename' will only update the remote's fetch refspec if it
looks like a default one. If the remote has no default fetch refspec,
as in

[remote "origin"]
    url = git://git.kernel.org/pub/scm/git/git.git
    fetch = +refs/heads/*:refs/remotes/upstream/*

we would not update the fetch refspec and even if there is a ref
called "refs/remotes/origin/master", we should not rename it, since it
was not created by fetching from the remote.

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