Build an "affected path" index when attaching diffs to revisions

Summary: See T262. This creates the index on the Differential side which we need in order to execute this query efficiently on the Diffusion side.

Also renames "DiffusionGitPathIDQuery" to "DiffusionPathIDQuery", this query object has nothing to do with git.

Test Plan: Attached top-level and sub-level diffs to revisions and verified they populated the table with sensible data.

Reviewers: bmaurer, aravindn, fmoo, jungejason, nh, tuomaspelkonen, aran

CC:

Differential Revision: 931