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 how it pulls file content. To make those changes easier to test, first start extracting this stuff so the code can be run with `bin/differential extract ...` instead of needing to do a bunch of more complicated setup steps.

Test Plan:
  - Ran `bin/differential extract ...` to extract diffs from commits.
  - Forced my way through the daemon workflow by faking out a bunch of flags, got a clean extract + attach + update. After this patch, this should rarely be necessary.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9319

Differential Revision: https://secure.phabricator.com/D14967