Return unloaded commit if it couldn't be enriched

What does this MR do?

When a commit could not be enriched, we want to return the original commit.

When calling Commit.lazy we get a BatchLoader instance that behaves like nil. So now we explicitly trigger the loading in batch and replace the commit with the original commit if it could not be loaded.

This makes sure the merge request view can still be loaded for merge requests for which the source branch became unavailable

#28649 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content