stream-clone: pre-indent some code This make the next changeset clearer.

local-clone: perform the hardlink/copy based from _entries_walk returns We previously used `_v2_walk`. However it is not bringing us much. So lets use the higher level function instead. This will offer us more flexibility with the `_v2_walk` function… like deleting it eventually.

store: cache the file_size when we get it from disk The point of caching `files` is to ensure consistency and avoiding redoing expensive work. So we cache the file_size once retrieved.

store: cache the `files()` return for store entries This make it more efficient to directly use the entries list to retrieve data in various location. It also make the entry record the file size it previously promissed to user code, especially the stream clone code.

stream-clone: introduce a richer TempCopyManager object This replace the previous `copy` callable with a richer object that allow access to the backup path. This will simplify the user code as they won't need to keep and pass around the backup path explicitly.

store: properly compute the targer_id of manifestlog in no-fncache walk Creating RevlogStoreEntry is good, but we need to drop the final `00manifest` part to create something correct.

store: do not drop the final `/` when creating manifestlog instance This bug, inherited from the upgrade code leads to the acces/creation of broken revlog with name `DIRECTORY00manifest.i` instead of `DIRECTORY/00manifest.i` We fix it in its own changeset to preserve the "pure code movement" aspect of the previous changesets.

store: add a `get_revlog_instance` method on revlog entries The upgrade code needs this a lot, and the stream code is about to needs it too. So we start by moving the upgrade code in a more generic location.

stream-clone: add a test that highlight crash on revlog splitting This has been a long running problem, we should have a tests for it.

stream-clone: remove unused code in test-clone-stream.t We are not using the extension we create inline, we are using `tests/testlib/ext-stream-clone-steps.py`. So let us deleted the unused version.