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.

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.

manifest: introduce a `read_any_fast_delta` method This method is a clearer semantic than `readbase` and `readfast` and will allow for more accurate optimization and usage. This is part of a wider series introducing such clearer method.

manifest: add many type annotations to the manifest module This help to clarify the API a bit, this caught various bug in the process and will help to catch more in the future. This also make large refactoring significantly simpler.

manifest: help pytype to understant `writesubtrees`'s `getnode` type Since we provide a default, the return of `_lazydirs.get` is cannot be None. We help pytype to understand that.

manifest: use explicit None checking in `_loaddifflazy` This helps pytype to understand what is going here with `v2` type.