storage_data: Expose revisions as model objects

This removes the invalid revision parents references.

This touches slightly the revision_*log tests because the arborescence revision, to be correct, is now a bit more involved.

from:

rev4 -is-child-of-> rev3 (stop)

becomes:

rev4 -is-child-of-> rev3 -> rev1, rev2; rev2 -> rev1

Related to #2494 (closed)

Depend on !858 (closed)

Test Plan

tox

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Migrated from D3586 (view on Phabricator)