The TransactionManager is currently packing all new and unreachable
objects into the packfile that gets committed as part of the
transaction. This is expensive as it does a full graph walk. It also
was only a temporary solution until we have the required functionality
in Git to only pack the new objects, and figure out the existing
objects a transaction depends on existing in the repository.

Git v2.44 includes the necessary functionality in
`rev-list --missing=print`. Previously it failed on missing commits, now
it prints them out. This commit wires the changes into TransactionManager
by doing the following.

When a transaction is about to commit, the TransactionManager does an
object walk in the quarantine directory only. It starts the walk from
the new reference tips in the transaction and the included objects. The
included objects are objects that are not reachable from the references
but we want to commit them into the repository nonetheless. This walks
the new objects in the quarantine and prints all of them out for packing.
As we perform the walk with only the quarantine configured, any objects
that are not in the quarantine are printed as missing objects. These objects
are considered the transaction's dependencies and they are verified to
exist in the repository prior to committing the transaction. The dependency
information can also be later used to prevent concurrent pruning operations
from pruning unreachable objects that are needed by some transactions.

This approach is significantly faster than the earlier approach as it
only walks and packs the new objects in the quarantine, and thus scales by
the size of the change, not by the size of the history.

Checking object existence requires a Git repository. The checks are done against
the staging repository which is a snapshot of the repository's latest state in the
partition plus the possible alternate configured for it during the transaction.
If the repository is being created and has no existing state, an empty repository
is used as the base state. The staging repository is used for both verifying
the dependency existence and the reference changes.

Git special cases the empty tree OID and considers it to be present in the
object database even if it hasn't explicitly been written there. This leads
to the object walk returning the empty tree OID which results it being
included in the pack. The tests were adjusted for this. In practice this
shouldn't have a big impact but we can later special case this and drop
the empty tree from the packs if necessary.

As transactions now require functionality in Git patched in bundled Git
but not yet release, we add version checks in main and TransactionManager
tests to guard against running with incorrect version.