FUSE: add history/by-page/ dir for revision objects

The dir in title should be populated with a paginated view of the ancestor commits of the given revision, to avoid exploding the number of entries in a single dir.

by-page/ should contain //fixed width// directories named 00, 01, 02, etc. Each of them should contain a maximum number of commits, let's say 10'000 as a starting value, which will give us ~100 pages for repos like the linux kernel.

The ordering, both within by-page/ and within sub-dirs (by-page/XY), should be topological. Ideally, it should be possible to have a oneliner that list //all// commits, e.g., ls -f history/by-page/*/ (note: I havent checked what is the ordering semantics of shell globbing *).

This should be supported only for revisions known to swh-graph (which is already the case for the parent history/ directory anyway).

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Migrated from T2772 (view on Phabricator)