Do not save duplicate notes when importing pipelines

What does this MR do and why?

This MR fixes a bug in Project Import/Export, where on import we create duplicate commit notes for every exported pipeline.

Example

original project	exported and re-imported project

This is happening because for every exported pipeline, we export all corresponding commit notes. Then, on import, we re-create exported notes for the same commit. But because there can be multiple pipelines for a single commit, we end up creating duplicate notes for every pipeline we exported.

In the example above, there are 39 pipelines for this commit & we end up with 39 mentioned in issue ... notes for this commit.

Mentions #365702 (closed)

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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