origin layer: retrieve multiple levels of revision history at once

Replace revision_get_parents with revision_get_some_outbound_edges,
which can optionally retrieve more levels of history than just a single
one. This allows us to do way fewer queries on the swh.graph or
swh.storage backend if the revision exists there.

The swh.storage backend does limited recursion, so we still process the
origin in multiple steps to fetch the whole history.