remote: let guess_remote_head() optionally return all matches

remote: let guess_remote_head() optionally return all matches

Determining HEAD is ambiguous since it is done by comparing SHA1s.

In the case of multiple matches we return refs/heads/master if it
matches, else we return the first match we encounter. builtin-remote
needs all matches returned to it, so add a flag for it to request such.

To be simple and consistent, the return value is now a copy (including
peer_ref) of the matching refs.

Originally contributed by Jeff King along with the prior commit as a
single patch.

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