Merge branch 'jc/read-tree-empty-with-m' into maint

"git read-tree -m" (no tree-ish) gave a nonsense suggestion "use
--empty if you want to clear the index".  With "-m", such a request
will still fail anyway, as you'd need to name at least one tree-ish
to be merged.

* jc/read-tree-empty-with-m:
  read-tree: "read-tree -m --empty" does not make sense

builtin/read-tree.c

@@ -210,7 +210,7 @@ int cmd_read_tree(int argc, const char **argv, const char *unused_prefix)
 			die("failed to unpack tree object %s", arg);
 		stage++;
 	}
-	if (nr_trees == 0 && !read_empty)
+	if (!nr_trees && !read_empty && !opts.merge)
 		warning("read-tree: emptying the index with no arguments is deprecated; use --empty");
 	else if (nr_trees > 0 && read_empty)
 		die("passing trees as arguments contradicts --empty");