rcutil: don't check if defaultrc/ is a directory -- we know it is `mercurial/defaultrc/` is a directory both in the Mercurial repo and once installed on a target platform. The directory was created in c4ce077588d0 (config: introduce "built-in" default configuration settings in default.d, 2014-09-04). That commit has some more information, but it still doesn't seem to say that `defaultrc/` (then called `default.d/`) could be a file. Perhaps the check was there to allow you to run the same code on an older install/repo? Differential Revision: https://phab.mercurial-scm.org/D7624