Skip new_blob_path in noteable data for Rapid Diffs

Stanislav Lashmanovrequested to merge
slashmanov/rapid-diffs-noteable-blob-path into master

What does this MR do and why?

The merge request page renders its full shell on every tab, including Pipelines and Commits, and serializes new_blob_path — a field only the legacy diffs empty state consumes. Building it requires a Gitaly ref-existence check on the source branch. With Rapid Diffs enabled the legacy diffs app never mounts, so nothing reads the field, and Rapid Diffs already builds its own "Create commit" link. This skips the exposure when Rapid Diffs is the current experience, dropping that Gitaly call from the page render with no user-visible change.

Reproduction steps

  1. Open any merge request page with Rapid Diffs enabled and confirm the page render no longer issues the ref-existence Gitaly call — check the performance bar's Gitaly panel for ref_service#list_refs triggered by the source branch check.
  2. Open a merge request with no changes and confirm the "There are no changes yet" empty state still offers "Create commit".
  3. Disable Rapid Diffs and confirm the legacy empty state still offers it too.

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