Move Mercurial discovery to PhabricatorRepositoryDiscoveryEngine

Summary: Ref T4068. Partly, this moves discovery to the more unit-testable PhabricatorRepositoryDiscoveryEngine. It also fixes some issues, see inlines.

Test Plan: In a Mercurial repository, ran `bin/repository discover --repair`, verified commits came out topographically sorted. Ran without `--repair` and in various other contexts, like with no commits to discover and some-but-not-all commits to discover.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4068

Differential Revision: https://secure.phabricator.com/D7518