Summary: Ref T9252. This is still crude in a few ways but basically works, at least for commits.
Test Plan:
- Made a build plan with just this build step.
- Ran `bin/harbormaster build --plan 10 ...` on a commit.
- It actually built a working copy, leased it, took no action, and released the lease. MAGIC~~~
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T9252
Differential Revision: https://secure.phabricator.com/D14160
