This PR contains a branch thicket on top of v2.55.0-rc1 (i.e. ready to
go upstream) to continue the bulk of the `unsigned long` -> `size_t`
transformation.
Since all of these changes have no impact on the currently-working
functionality for <4GB objects/packs/clones (modulo bugs, that is 😄), I
would like to merge this before v2.55.0-rc2, still: The risk of
introducing a regression is negligible, the chance for fixing the
majority of problems with large clones is high.