Avoid loading CommitsMetadata in MergeRequest#changed_paths

What does this MR do and why?

MergeRequest#changed_paths passed the full CommitCollection (from MergeRequest#commits) to find_changed_paths. Building that collection triggers MergeRequestDiff#load_commits → diff_commits.with_users which loads every CommitsMetadata AR object (including message, authored_date, committed_date, sha, plus two DiffCommitUser joins) for the entire diff. All of that was just to produce SHAs, since find_changed_paths_request only reads object.sha.

Changes:

1. commit_service.rb- add when String to find_changed_paths_request

SHA strings (exactly 40 or 64 hex chars, validated via Gitlab::Git.commit_id?) are now turned into CommitRequest objects directly. Non-hex strings still fail commit_id? and are filtered out, so the existing "invalid objects" test is unaffected.

2. merge_request.rb - use commit_shas instead of commits when FF optimised_commits_for_mr_changed_paths is enabled.

commit_shas delegates to merge_request_diff.commit_shas which, when the association isn't loaded, runs commit_shas_from_metadata, a single SQL query that returns only hex SHA strings. Which avoids loading CommitsMetadata and DiffCommitUser objects.

Tests updated:

• merge_request_spec.rb: #changed_paths now stubs commit_shas instead of commits

• commit_service_spec.rb: new context showing SHA strings passed directly produce identical Gitaly requests to Commit objects

References

related to #592062

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