Add basic Mercurial support to the watchman repo

Summary: Add an hgignore and arc support.

Test Plan: Check `arc which` in both hg and git.

Reviewers: wez

Reviewed By: wez

CC: fugalh

Differential Revision: https://phabricator.fb.com/D863836

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extra : amend_source : e2d5d22aaa50bd8ab18338007061b71f9cc1543b