pack-objects: introduce '--stdin-packs=follow'

When invoked with '--stdin-packs', pack-objects will generate a pack
which contains the objects found in the "included" packs, less any
objects from "excluded" packs.

Packs that exist in the repository but weren't specified as either
included or excluded are in practice treated like the latter, at least
in the sense that pack-objects won't include objects from those packs.
This behavior forces us to include any cruft pack(s) in a repository's
multi-pack index for the reasons described in ddee3703b3
(builtin/repack.c: add cruft packs to MIDX during geometric repack,
2022-05-20).

The full details are in ddee3703b3, but the gist is if you
have a once-unreachable object in a cruft pack which later becomes
reachable via one or more commits in a pack generated with
'--stdin-packs', you *have* to include that object in the MIDX via the
copy in the cruft pack, otherwise we cannot generate reachability
bitmaps for any commits which reach that object.

Note that the traversal here is best-effort, similar to the existing
traversal which provides name-hash hints. This means that the object
traversal may hand us back a blob that does not actually exist. We
*won't* see missing trees/commits with 'ignore_missing_links' because:

 - missing commit parents are discarded at the commit traversal stage by
   revision.c::process_parents()

 - missing tag objects are discarded by revision.c::handle_commit()

 - missing tree objects are discarded by the list-objects code in
   list-objects.c::process_tree()

But we have to handle potentially-missing blobs specially by making a
separate check to ensure they exist in the repository. Failing to do so
would mean that we'd add an object to the packing list which doesn't
actually exist, rendering us unable to write out the pack.

This prepares us for new repacking behavior which will "resurrect"
objects found in cruft or otherwise unspecified packs when generating
new packs. In the context of geometric repacking, this may be used to
maintain a sequence of geometrically-repacked packs, the union of which
is closed under reachability, even in the case described earlier.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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