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>

remote.h

@@ -139,13 +139,12 @@ int stat_tracking_info(struct branch *branch, int *num_ours, int *num_theirs);
 int format_tracking_info(struct branch *branch, struct strbuf *sb);
 
 struct ref *get_local_heads(void);
-/*
- * Look in refs for HEAD. Then look for a matching SHA1 in mapped_refs,
- * first checking if refs/heads/master matches. Return NULL if nothing matches
- * or if there is no HEAD in refs. remote_head_p is assigned HEAD if not NULL.
- */
-const struct ref *guess_remote_head(const struct ref *refs,
-				    const struct ref *mapped_refs,
-				    const struct ref **remote_head_p);
 
+/*
+ * Look for a ref in refs whose SHA1 matches head, first checking if
+ * refs/heads/master matches. Return NULL if nothing matches or if head
+ * is NULL.
+ */
+const struct ref *guess_remote_head(const struct ref *head,
+				    const struct ref *refs);
 #endif