Feature flag: Preload Gitaly data in ensure_merge_request_diff to reduce idle in transaction

Summary

Feature flag preload_gitaly_in_ensure_mr_diff for rolling out the change to preload Gitaly data in ensure_merge_request_diff.

This moves Gitaly RPC calls outside the database transaction boundary during new merge request creation in NewMergeRequestWorker, reducing idle-in-transaction time.

Related MR

!225840 (merged)