remote: simplify guess_remote_head()

remote: simplify guess_remote_head()

This function had complications which made it hard to extend.

- It used to do two things: find the HEAD ref, and then find a
  matching ref, optionally returning the former via assignment to a
  passed-in pointer. Since finding HEAD is a one-liner, just have a
  caller do it themselves and pass it as an argument.

- It used to manually search through the ref list for
  refs/heads/master; this can be a one-line call to
  find_ref_by_name.

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

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