refs: convert MERGE_AUTOSTASH to become a normal pseudo-ref

Similar to the preceding conversion of the AUTO_MERGE pseudo-ref, let's
convert the MERGE_AUTOSTASH ref to become a normal pseudo-ref as well.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.c

@@ -1875,7 +1875,6 @@ static int is_special_ref(const char *refname)
 	 */
 	static const char * const special_refs[] = {
 		"FETCH_HEAD",
-		"MERGE_AUTOSTASH",
 		"MERGE_HEAD",
 	};
 	size_t i;