Gitaly's write-ahead log needs to include pack files with the log
entries in order to log the objects the transactions introduce. Along
the new objects, the pack files must also include objects that are
made reachable in the transaction. This guarantees the pack files will
apply later from the log even if some of the unreachable objects were
pruned while the pack file was sitting in the log. This commit implements
WalkUnreachableObjects to help with computing such a pack file. It takes
a list of new heads in the transaction and outputs a list of objects that
are reachable from these heads but not from the existing references in the
repository. When this is ran on a localrepo instance that has been
configured with the transaction's quarantine directory as an alternate,
it yields the list of new and newly reachable objects compared to the
current set of references.