in_memory: Make origin-visit-status-add respect "on conflict ignore" policy
Prior to this commit, that behavior was not properly tested and inconsistent between backends. All backends except in-memory were respecting it.
This commit aligns the in-memory backend implementation and test it.
It's the source of the current failure in !933 (closed)
Depends on !419 (closed) Related to T2310
Test Plan
tox
Migrated from D3278 (view on Phabricator)