remote: make match_refs() copy src ref before assigning to peer_ref

remote: make match_refs() copy src ref before assigning to peer_ref

In some instances, match_refs() sets the peer_ref field of refs in the
dst list such that it points to a ref in the src list. This prevents
callers from freeing both the src and dst lists, as doing so would cause
a double-free since free_refs() frees the peer_ref.

As well, the following configuration causes two refs in the dst list to
have the same peer_ref, which can also lead to a double-free:

  push = refs/heads/master:refs/heads/backup
  push = refs/heads/master:refs/heads/master

Existing callers of match_heads() call it only once and then terminate,
w/o ever bothering to free the src or dst lists, so this is not
currently a problem.

This patch modifies match_refs() to first copy any refs it plucks from
the src list before assigning them as a peer_ref. This allows
builtin-remote, a future caller, to free the src and dst lists.

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>