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>

builtin-clone.c

@@ -509,8 +509,8 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 
 		mapped_refs = write_remote_refs(refs, &refspec, reflog_msg.buf);
 
-		head_points_at = guess_remote_head(refs, mapped_refs,
-						   &remote_head);
+		remote_head = find_ref_by_name(refs, "HEAD");
+		head_points_at = guess_remote_head(remote_head, mapped_refs);
 	}
 	else {
 		warning("You appear to have cloned an empty repository.");