Summary:
With the change, a new delegate method allows a receiver to alter a new (proposed) shadow tree with another tree by returning the altered tree.
We will it use in future diffs to implement Commit Hooks.
Changelog: [Internal] Fabric-specific internal change.
Reviewed By: sammy-SC
Differential Revision: D25221313
fbshipit-source-id: 9f83577d862b713fff71fa365ce660cc1de87c84