Reduce N+1 project queries in merge_request_diff

What does this MR do and why?

While I was looking at reducing the number of queries for !210454 (merged) to address failing spec, I noticed lots of project lookups coming from this feature flag check as it was called for each diff.

The original change had been introduced in !203146 (merged)

Query count:

• Before: 257
• After: 217

NOTE: I intentionally didn't reduce the query count linit in the spec and I'll update it later once !210454 (merged) gets merged.

References

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Before	After

How to set up and validate locally

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