merge: a random object may not necssarily be a commit

The user could have said "git merge $(git rev-parse v1.0.0)"; we
shouldn't mark it as "Merge commit '15999998fb...'" as the merge
name, even though such an invocation might be crazy.

We could even read the "tag " header from the tag object and replace
the object name the user gave us, but let's not lose the information
by doing so, at least not yet.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge.c

@@ -516,6 +516,19 @@ static void merge_name(const char *remote, struct strbuf *msg)
 		strbuf_release(&line);
 		goto cleanup;
 	}
+
+	if (remote_head->util) {
+		struct merge_remote_desc *desc;
+		desc = merge_remote_util(remote_head);
+		if (desc && desc->obj && desc->obj->type == OBJ_TAG) {
+			strbuf_addf(msg, "%s\t\t%s '%s'\n",
+				    sha1_to_hex(desc->obj->sha1),
+				    typename(desc->obj->type),
+				    remote);
+			goto cleanup;
+		}
+	}
+
 	strbuf_addf(msg, "%s\t\tcommit '%s'\n",
 		sha1_to_hex(remote_head->object.sha1), remote);
 cleanup: