Use file-by-file diff highlight cache when requesting diffs in batch

Problem

Our current implementation of /diffs_batch doesn't use the diff highlight cache given we don't support reading the cache in a file-by-file (or batch) basis, which is being handled at #30550 (closed).

Proposal

Use the cache interface (e.g. read_multiple / write_multiple (if the cache is not in place)) in order to populate the diff highlight per batch at /diffs_batch endpoint.