Reset Drydock git working copies better

Summary: Ref T9252. We're currently resetting to the local branch, but should be resetting to the origin branch.

Test Plan: Restarted a build, had it run git show, saw proper HEAD.

Reviewers: chad

Reviewed By: chad

Subscribers: hach-que

Maniphest Tasks: T9252

Differential Revision: https://secure.phabricator.com/D14194