Support disordered directory entries in git

Some git directories don't have their entries in the right order, which is an issue for us because:

swh-model reorders them before checksuming
the postgresql storage does not preserve the relative order of file and dir entries
the cassandra storage does not preserve order at all (they are sorted by null-padded name)

Relevant issue to consider to regarding Cassandra: swh/devel/swh-storage#3582 (closed)

Migrated from T3595 (view on Phabricator)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information