Support automatic "Depends On" detection in Mercurial

Summary: Depends on D18651. Allows Mercurial to auto-detect depends-on revisions.

Test Plan: See D18652 and D18653. Didn't actually test the empty repository states but they're probably okay. The experimental branch is an untamed wild. ¯\_(ツ)_/¯

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18654