R1:09c3c7d87931

Add support to marking commits as UNREACHABLE for Mercurial

Summary:
When previously known commits have been destroyed in a Mercurial repository, Phabricator does not end up marking the commits as unreachable. This results in daemon tasks which continuously fail and retry.

This updates `PhabricatorRepositoryDiscoveryEngine` and `PhabricatorManagementRepositoryMarkReachableWorkflow` to include support of marking commits as unreachable for Mercurial repositories.

The `PhabricatorMercurialGraphStream` also…