merge_file(): add comment explaining behavior wrt conflict style

The merge_file() function is a helper for ‘git read-tree’, which does
not respect the merge.conflictstyle option, so there is no need to
worry about what ancestor_name it should pass to ll_merge().  Add a
comment to this effect.

Signed-off-by: Jonathan Nieder <jrnieder@mgila.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

merge-file.c

@@ -30,6 +30,12 @@ static void *three_way_filemerge(const char *path, mmfile_t *base, mmfile_t *our
 	int merge_status;
 	mmbuffer_t res;
 
+	/*
+	 * This function is only used by cmd_merge_tree, which
+	 * does not respect the merge.conflictstyle option.
+	 * There is no need to worry about a label for the
+	 * common ancestor.
+	 */
 	merge_status = ll_merge(&res, path, base, NULL,
 				our, ".our", their, ".their", 0);
 	if (merge_status < 0)