manifest: use `read_any_fast_delta` during shallow prefetch's We now have a better function with a clear semantic. This simplify the usage in the remotefilelog code.

manifest: use `read_any_fast_delta` during remotefilelog's repack We now have a better function with a clear semantic. This simplify the usage in the remotefilelog code.

manifest: use read_any_fast_delta in changectx The new API is clearer but also more expressive. It allow to detect case where we did return a full read and populated the associated cache. Saving time!

manifest: allow skipping valid_bases argument to `read_any_fast_delta` In some case it make sens to just want a delta. So we update the API to support this.