Give all commands from DiffusionCommandEngine a default 15 minute timeout

Summary:
Ref T13108. See PHI364. See the task and issue for discussion.

If a git fetch during synchronization hangs, the whole node currently hangs. While the causes of a git fetch hang aren't clear, we don't expect synchronization to ever reasonably take more than 15 minutes, so add a default timeout.

Test Plan: Will deploy and observe; this is difficult to reproduce or test directly.

Maniphest Tasks: T13108

Differential Revision: https://secure.phabricator.com/D19235