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>
diff --git a/builtin/commit.c b/builtin/commit.c
index 65196a2..6d1fa71 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1877,7 +1877,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 				     &oid, flags);
 	}
 
-	apply_autostash(git_path_merge_autostash(the_repository));
+	apply_autostash_ref(the_repository, "MERGE_AUTOSTASH");
 
 cleanup:
 	strbuf_release(&author_ident);