R1:7150aa8e192e

Use Conduit in PhabricatorRepositoryGitCommitChangeParserWorker

Summary:
Ref T2783. This allows this worker to run on a machine different to the one that stores the repository, by routing the execution of Git over Conduit calls.

This API method is super gross, but fixing it isn't straightforward and it runs into other complicated considerations. We can fix it later; for now, just define it as "internal" to limit how much mess this creates.

"Internal" methods do not appear on the console.

Test Plan…