R1:bd923d1ce091

Provide an explicit "-R" flag to "hg serve"

Summary:
See <https://discourse.phabricator-community.org/t/unable-to-use-current-mercurial-on-debian-stretch/391>.

The Mercurial commit is helpful in particular: <https://www.mercurial-scm.org/repo/hg/rev/77eaf9539499>

We weren't vulnerable to the security issue (users can not control any part of the command) but pass the working directory explicitly to get past the new safety check.

I left `setCWD()` in place (a few lines below) just because it can't hurt…