R1:d735c7adf2d5

Allow Harbormaster to run commands on Drydock working copies

Summary: Ref T9252. This mostly cleans up future and log handling, and edges us closer to being able to do useful work with Harbormaster / Drydock.

Test Plan:
- Added a "Run `ls -alh`" step to my trivial build plan.
- Ran it a bunch of times.
- Worked great.
- Also did an HTTP plan.

{F835227}

{F835228}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9252

Differential Revision: https://secure.phabricator.com/D14161