Add a pipeline creation request check to diff_head_pipeline_considered_in_progress?

What does this MR do and why?

In this MR, we return sooner from diff_head_pipeline_considered_in_progress?, if a pipeline creation is in progress.

The Problem

We're allowing brief moments of mergeability between pipeline executions when we shouldn't be. See details in the parent issue.

Related to #581480 (closed)