manifest: use the `read_delta_parents` method Let's use the more accurate API.

manifest: use `read_delta_parents` when adjusting linkrev in remotefile Let's use the more accurate API.

manifest: introduce a `read_delta_parents` method This new method have a clearer semantic and can be used by code that need this semantic. This should avoid bugs, allow for more targeted optimisation, and provide a clearer API. Users will be updated in subsequent changesets. This is also part of the wider effort to clarify and fix this API. one more method coming.

manifest: use `read_any_fast_delta` for tag rev cache computation This will have the benefit of using the fast path more often, and being (a bit) less buggy. See inline comment for details.

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.