Merge request pipelines are a valid source context

What does this MR do and why?

This MR fixes a bug where merge request pipelines would not properly trigger ingestion on a tracked branch (where the source branch of the MR is tracked). The root of the issue is how we are attempting to find a tracked context from a pipeline - if it's a merge request then we need to resolve the correct reference indirectly through the merge request object.

MR acceptance checklist

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