Prevent "Wait for Build Commits" from creating billions of logs

Summary:
Resolves T5987. This build step was at some point converted to use yielding, which meant that whenever the build step executes it will create a new log. This checks to see if there is an existing log before creating a new one and uses that instead.

Long term we're going to need some way of attaching data to PhabricatorWorkerYieldException that can be read when the build step starts again; this will allow us to move more build steps off while (...) { ... sleep(X); } loops and onto yielding.

Test Plan: Tested locally.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley

Maniphest Tasks: T5987

Differential Revision: https://secure.phabricator.com/D10383