object-store: remove global array of cached objects

Cached objects are virtual objects that can be set up without writing
anything into the object store directly, which is used by git-blame(1)
to create fake commits for the working tree.

These cached objects are stored in a global variable, which is another
roadblock for libification of the object subsystem. Refactor the code so
that we instead store the array as part of the raw object store.

This refactoring raises the question whether virtual objects should
really be specific to a single repository (or rather a single object
store). Hypothetical usecases might for example span across submodules,
and here it may or may not be the right thing to provide virtual objects
across submodule boundaries.

The only existing usecase is git-blame(1) though, which does not know to
blame across submodule boundaries in the first place. As such, storing
these objects both globally and per-repository would achieve the same
result right now. But arguably, if we learned to blame across submodule
boundaries, we would likely want to create separate fare working tree
commits for each of the submodules so that the user can learn which
worktree a specific uncommitted change belongs to. And even if we would
want to create the same fake commit for each of the submodules we could
do that when storing separate virtual objects per object store.

While this is all rather hypothetical, the takeaway is that handling
virtual objects per-object store gives us more flexibility compared to
storing them globally. In a hypothetical future where we have achieved
full libification one might be able to handle unrelated repositories in
a single process, where the state of one repository should not have an
impact on the state of another repository. As such, storing these cached
objects per object store will enable more usecases and should lead to
less surprising outcomes overall.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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