Use diff highlight cache for diffs batch loading
What does this MR do?
This extends the work done at !19917 (merged) in order to use the new cache for diffs batch requests.
Slight performance improvements can be seen locally, from a MR with 304 changes taking around 9 seconds to serialize diff files, it now takes below 6 seconds. So it's around 30% improvement.
Edited by Oswaldo Ferreira