Summary: Removes `TreeDiffer` class and passes `DiffContext` through standalone `TreeDiffer` functions as first argument as per comment on D17400466 for setup for processing gitignores in the `TreeDiffer` codepath. (also this allows for easy implementation of short circut of `future_getScmStatusBetweenRevisions` similar to D17531102)
Reviewed By: chadaustin
Differential Revision: D17717977
fbshipit-source-id: d480d212474bd80aeac9cd9bb901f97562b62b13
