R1:79fdf5c12707

Separate changeset analysis code from DifferentialDiff and provide a standalone `rebuild-changesets` workflow

Summary:
Ref T13137. The "analyze/cache data about changesets" step is becoming more involved. We recently added detection for generated code to support "Ignore generated changes" in Owners, and I now plan to hash the new file content so we can hide changes which have no effect.

Before adding this new hashing step, pull the "detect copied code" and "detect generated code" stuff out and move them…