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` to include support of marking commits as unreachable for Mercurial repositories.
The `PhabricatorMercurialGraphStream` also needed updated to support a stream with no starting commit.
Refs T13634