R1:f015cb50fe6a

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…