rust-discovery: read the index from a repo passed at init This makes the API of the Rust PartialDiscovery object now the same (or rather a subset) of the Python object, hence easier to control through module policy down the road. Differential Revision: https://phab.mercurial-scm.org/D6517

rust-discovery: accept the new 'respectsize' init arg At this stage, we don't do anything about it: it will be meaningful in sampling methods that aren't implemented yet. Differential Revision: https://phab.mercurial-scm.org/D6516

merge with stable

unshelve: forget unknown files after a partial unshelve This is a follow-up patch to 6957f7b93e03. This allows hg to forget unknown files after a partial unshelve. Differential Revision: https://phab.mercurial-scm.org/D6724

flagutil: move addflagprocessor to the new module (API)

flagutil: move insertflagprocessor to the new module (API)

flagutil: move REVIDX_KNOWN_FLAGS source of truth in flagutil (API) Since REVIDX_KNOWN_FLAGS is "not really a constant" (extension can update it) and python integer,... it needs to be the responsability of a single module and always accessed through the module. We update all the user and move the source of truth in flagutil.