move locate_head() to remote.c

Move locate_head() to remote.c and rename it to guess_remote_head() to
more accurately reflect what it does. This is in preparation for being
able to call it from builtin-remote.c

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

remote.c

@@ -1451,3 +1451,40 @@ struct ref *get_local_heads(void)
 	for_each_ref(one_local_ref, &local_tail);
 	return local_refs;
 }
+
+const struct ref *guess_remote_head(const struct ref *refs,
+				    const struct ref *mapped_refs,
+				    const struct ref **remote_head_p)
+{
+	const struct ref *remote_head = NULL;
+	const struct ref *remote_master = NULL;
+	const struct ref *r;
+	for (r = refs; r; r = r->next)
+		if (!strcmp(r->name, "HEAD"))
+			remote_head = r;
+
+	for (r = mapped_refs; r; r = r->next)
+		if (!strcmp(r->name, "refs/heads/master"))
+			remote_master = r;
+
+	if (remote_head_p)
+		*remote_head_p = remote_head;
+
+	/* If there's no HEAD value at all, never mind. */
+	if (!remote_head)
+		return NULL;
+
+	/* If refs/heads/master could be right, it is. */
+	if (remote_master && !hashcmp(remote_master->old_sha1,
+				      remote_head->old_sha1))
+		return remote_master;
+
+	/* Look for another ref that points there */
+	for (r = mapped_refs; r; r = r->next)
+		if (r != remote_head &&
+		    !hashcmp(r->old_sha1, remote_head->old_sha1))
+			return r;
+
+	/* Nothing is the same */
+	return NULL;
+}