R1:0dd947cceddc

Move diff extraction from commits to a separate test with a CLI command

Summary:
Ref T9319. When we discover a commit, we sometimes update the corresponding revision with a "this is the actual committed change" diff and send out a link to the changes between review and commit.

This is currently very difficult to test, because it only happens the first time and you have to either go set up a bunch of objects or add a bunch of special casing to the parser to hit the workflow.

I'm making some changes to…