Split mercurial parsing and API

Summary: Move code to actually parse "hg" output into a separate class with some
tests, so I can reuse it in the import scripts. We should probably do this for
Git/SVN at some point, too.

Test Plan: Ran unit tests, used this class in Phabricator importers, grepped for
calls to removed private methods.

Reviewers: Makinde, jungejason, nh, tuomaspelkonen, aran

Reviewed By: Makinde

CC: aran, Makinde

Differential Revision: 942