Make "git-remote prune" delete refs according to fetch specs

Make "git-remote prune" delete refs according to fetch specs

A remote may be configured to fetch into tracking branches that
do not match the remote name.  For example a user may have created
extra remotes that will fetch to the same tracking branch namespace,
but from different URLs:

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

  [remote "alt"]
    url = git://repo.or.cz/alt-git.git
    fetch = refs/heads/*:refs/remotes/origin/*

When running `git remote prune alt` we expect stale branches to
be removed from "refs/remotes/origin/*" and not from the unused
namespace of "refs/remotes/alt/*".

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>