merge.c

	if (!common)
		; /* No common ancestors found. We need a real merge. */
	else if (!remoteheads->next && !common->next &&
			common->item == remoteheads->item) {
		/*
		 * If head can reach all the merge then we are up to date.
		 * but first the most common case of merging one remote.
		 */
		finish_up_to_date("Already up-to-date.");
		return 0;
	} else if (allow_fast_forward && !remoteheads->next &&
			!common->next &&
			!hashcmp(common->item->object.sha1, head)) {
		/* Again the most common case of merging one remote. */
		struct strbuf msg = STRBUF_INIT;
		struct object *o;
		char hex[41];

		strcpy(hex, find_unique_abbrev(head, DEFAULT_ABBREV));

		if (verbosity >= 0)
			printf("Updating %s..%s\n",
				hex,
				find_unique_abbrev(remoteheads->item->object.sha1,
				DEFAULT_ABBREV));
		strbuf_addstr(&msg, "Fast-forward");
		if (have_message)
			strbuf_addstr(&msg,
				" (no commit created; -m option ignored)");
		o = peel_to_type(sha1_to_hex(remoteheads->item->object.sha1),
			0, NULL, OBJ_COMMIT);
		if (!o)
			return 1;

		if (checkout_fast_forward(head, remoteheads->item->object.sha1))
			return 1;

		finish(o->sha1, msg.buf);
		drop_save();
		return 0;
	} else if (!remoteheads->next && common->next)
		;
		/*
		 * We are not doing octopus and not fast-forward.  Need
		 * a real merge.
		 */
	else if (!remoteheads->next && !common->next && option_commit) {
		/*
		 * We are not doing octopus, not fast-forward, and have
		 * only one common.
		 */
		refresh_cache(REFRESH_QUIET);
		if (allow_trivial && !fast_forward_only) {
			/* See if it is really trivial. */
			git_committer_info(IDENT_ERROR_ON_NO_NAME);
			printf("Trying really trivial in-index merge...\n");
			if (!read_tree_trivial(common->item->object.sha1,
					head, remoteheads->item->object.sha1))
				return merge_trivial();
			printf("Nope.\n");
		}
	} else {
		/*
		 * An octopus.  If we can reach all the remote we are up
		 * to date.
		 */
		int up_to_date = 1;
		struct commit_list *j;

		for (j = remoteheads; j; j = j->next) {
			struct commit_list *common_one;

			/*
			 * Here we *have* to calculate the individual
			 * merge_bases again, otherwise "git merge HEAD^
			 * HEAD^^" would be missed.
			 */
			common_one = get_merge_bases(lookup_commit(head),
				j->item, 1);
			if (hashcmp(common_one->item->object.sha1,
				j->item->object.sha1)) {
				up_to_date = 0;
				break;
			}
		}
		if (up_to_date) {
			finish_up_to_date("Already up-to-date. Yeeah!");
			return 0;
		}
	}