R1:702f073f0a21

Fix some logic in WaitForPreviousBuildStep

Summary: Fixes T5062. See inlines.

Test Plan: Did not test whatsoever.

Reviewers: hach-que

Reviewed By: hach-que

Subscribers: epriestley

Maniphest Tasks: T5062

Differential Revision: https://secure.phabricator.com/D9132