Merge branch 'ar/diff-index-merge-base-fix'

"git diff --merge-base X other args..." insisted that X must be a
commit and errored out when given an annotated tag that peels to a
commit, but we only need it to be a committish.  This has been
corrected.

* ar/diff-index-merge-base-fix:
  diff: fix --merge-base with annotated tags

diff-lib.c

@@ -571,8 +571,6 @@ void diff_get_merge_base(const struct rev_info *revs, struct object_id *mb)
 		struct object *obj = revs->pending.objects[i].item;
 		if (obj->flags)
 			die(_("--merge-base does not work with ranges"));
-		if (obj->type != OBJ_COMMIT)
-			die(_("--merge-base only works with commits"));
 	}
 
 	/*