Fix Rapid Diffs version self-compare empty state

Stanislav Lashmanovrequested to merge
slashmanov/rapid-diffs-fix-version-self-compare into master

What does this MR do?

When comparing a merge request version against itself (or against a newer version) in Rapid Diffs, the diff resolver rendered the selected version's full diff instead of the No changes between X and Y empty state.

The legacy diffs controller (find_merge_request_diff_compare) collapses the comparison onto the diff's own head when start_sha does not match a strictly-older version. Gitlab::MergeRequests::DiffResolver was missing this, so it fell through to the full version diff.

This MR:

  • Mirrors the legacy self-compare behaviour in DiffResolver (empty diff → empty state).
  • Exposes the effective start_sha from the resolver and uses it for the target version dropdown, so the compared version resets to match the selected version.
  • Re-enables the two version self-compare feature specs skipped during the Rapid Diffs migration.

Part of the Rapid Diffs default-on migration follow-up fixes.

How to reproduce

  1. Open a merge request with multiple diff versions with Rapid Diffs.
  2. In the Compare (target) dropdown pick an older version.
  3. In the version (source) dropdown pick the same or an older version.
  4. Expected: No changes between <source> and <target> and the compare dropdown resets to the same version.

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