merge: virtual commit should be last argument to merge-base

Our virtual commit must be the last argument to merge-base: since our
algorithm pushes _both_ parents of the virtual commit, it needs to be
the last argument, since merge-base:

> Given three commits A, B and C, git merge-base A B C will compute the
> merge base between A and a hypothetical commit M

We want to calculate the merge base between the actual commit ("two")
and the virtual commit ("one") - since one actually pushes its parents
to the merge-base calculation, we need to calculate the merge base of
"two" and the parents of one.

src/merge.c

@@ -2271,8 +2271,14 @@ static int compute_base(
 	if (given_opts)
 		memcpy(&opts, given_opts, sizeof(git_merge_options));
 
-	if ((error = insert_head_ids(&head_ids, one)) < 0 ||
-		(error = insert_head_ids(&head_ids, two)) < 0 ||
+	/* With more than two commits, merge_bases_many finds the base of
+	 * the first commit and a hypothetical merge of the others. Since
+	 * "one" may itself be a virtual commit, which insert_head_ids
+	 * substitutes multiple ancestors for, it needs to be added
+	 * after "two" which is always a single real commit.
+	 */
+	if ((error = insert_head_ids(&head_ids, two)) < 0 ||
+		(error = insert_head_ids(&head_ids, one)) < 0 ||
 		(error = git_merge_bases_many(&bases, repo,
 			head_ids.size, head_ids.ptr)) < 0)
 		goto done;