Add a pipeline creation request check to diff_head_pipeline_considered_in_progress?

Daniel Prauserequested to merge
581480-add-a-pipelinecreation-request-check-to-diff_head_pipeline_considered_in_progress into master

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)

Edited by Daniel Prause

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