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