notes: allow merging from arbitrary references

Create a new expansion function, expand_loose_notes_ref which will first
check whether the ref can be found using get_sha1. If it can't be found
then it will fallback to using expand_notes_ref. The content of the
strbuf will not be changed if the notes ref can be located using
get_sha1. Otherwise, it may be updated as done by expand_notes_ref.

Since we now support merging from non-notes refs, remove the test case
associated with that behavior. Add a test case for merging from a
non-notes ref.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Reviewed-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

notes.c

@@ -1303,3 +1303,13 @@ void expand_notes_ref(struct strbuf *sb)
 	else
 		strbuf_insert(sb, 0, "refs/notes/", 11);
 }
+
+void expand_loose_notes_ref(struct strbuf *sb)
+{
+	unsigned char object[20];
+
+	if (get_sha1(sb->buf, object)) {
+		/* fallback to expand_notes_ref */
+		expand_notes_ref(sb);
+	}
+}