Fix rules:changes net diff for reverted files in MR pipelines

What does this MR do and why?

Fixes rules:changes firing for files that were changed and then reverted within an MR (when compare_to isn't set).

• MergeRequest#changed_paths used to union the changes from every commit in the MR, so a file touched in one commit and reverted in another stayed "changed" forever and kept triggering rules:changes
• Now it computes a single net tree diff between the MR's base and head (FindChangedPaths with a base..head tree pair), so reverted files cancel out. This matches the documented behavior and the existing compare_to path
• Still uses the same git-diff-tree Gitaly RPC, so it's not subject to diff display limits (keeps the #433257 (closed) fix intact) and doesn't go back to the slow diff_stats path.
• Behind the mr_changed_paths_net_diff feature flag

References

Relates to #597640

How to set up and validate locally

1. Turn :mr_changed_paths_net_diff feature on
2. In a test project, create my_file.txt on master.
3. Add .gitlab-ci.yml:

rules-changes-test:
  script: echo "foo bar"
  rules:
    - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
      changes: [my_file.txt]

3. Branch off, open an MR, then push a commit changing my_file.txt, then a commit reverting it to the original content.
   1. The latest MR pipeline should not contain rules-changes-test.
4. Turn feature off and retest, the job shows up

MR acceptance checklist

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Related to #597640