Provide a Differential Revision query class for affected paths

Summary:
For T262, we need to query for revisions by affected path.

We currently have a class called "DifferentialRevisionListData" but it's sort of
nasty and it would have been really cumbersome to add this query to it.

Instead, this provides a query object more in line with ManiphestTaskQuery,
which I'm pretty happy with. I'd eventually like to get rid of
DifferentialRevisionListData but it's used in a couple of places right now.

Test Plan: Used phpsh to execute queries, got back apparently-sensible result
sets.

Reviewers: jungejason, nh, tuomaspelkonen, aran

Reviewed By: jungejason

CC: aran, jungejason, epriestley

Differential Revision: 978