Ref T1751. Extends revert edges by adding new edge types:

• DifferentialRevisionRevertedByCommitEdgeType
• DiffusionCommitRevertsRevisionEdgeType
• DiffusionCommitRevertsTasksEdgeType
• ManiphestTaskRevertedByCommitEdgeType

This means that when a commit is reverted, an edge (and transaction) is created on any tasks/revisions which are associated with the reverted commit(s).

Reverted a commit which contained Ref Tx and Fixes Ty and also mentioned Tz. Saw reverting edges created for Tx and Ty.