Swap "-R" and "serve" argument order for Mercurial

Summary: See https://discourse.phabricator-community.org/t/unable-to-use-current-mercurial-on-debian-stretch/391/13. I missed that -R is order-sensitive.

Test Plan: Verified both orders work on 3.5.2.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18616