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: Ran bin/repository reparse --change <commit> --trace on several commits, saw daemons make a Conduit call instead of running a git command.

Reviewers: hach-que, chad

Reviewed By: chad

Subscribers: joshuaspence, Korvin, epriestley

Maniphest Tasks: T2783

Differential Revision: https://secure.phabricator.com/D11874