Preload Gitaly data in ensure_merge_request_diff

Summary

Pass preload_gitaly: true in ensure_merge_request_diff so that Gitaly RPC calls (fetching commits and diffs) happen before the database transaction, reducing idle-in-transaction time during new merge request creation.

This applies the same optimization already used in ReloadDiffsService and ReloadMergeHeadDiffService to the initial diff creation path in NewMergeRequestWorker → AfterCreateService.

Details

In the production MR creation flow:

1. MergeRequests::CreateService sets skip_ensure_merge_request_diff = true, so the after_create callback on MergeRequest is skipped during save!.
2. After the transaction commits, NewMergeRequestWorker enqueues and calls AfterCreateService#execute.
3. AfterCreateService calls merge_request.ensure_merge_request_diff directly — outside any DB transaction.
4. ensure_merge_request_diff calls create_merge_request_diff(preload_gitaly: true).

With the preload_gitaly flag:

• A new MergeRequestDiff is built (not yet persisted).
• preload_gitaly_data is called, which fetches commits and diffs from Gitaly via reversed_compare_commits_preloaded and compare_diffs_preloaded — both strong_memoized. This happens outside any transaction.
• save! is then called, which opens a transaction and triggers the after_create :save_git_content callback. Inside save_git_content, save_commits and save_diffs call the same memoized methods, so they use cached data — no Gitaly RPCs occur inside the transaction.

Without the flag (current behavior), merge_request_diffs.create! opens a transaction first, then the after_create :save_git_content callback makes Gitaly RPCs inside the transaction, causing idle-in-transaction while waiting on Gitaly.

Test

Added specs to verify ensure_merge_request_diff passes preload_gitaly: true (or false) to create_merge_request_diff depending on the feature flag state.