backfill: Reverse flawed logic in SnapshotBranch generation
The previous code would nullify all non-null branches, and try to create a SnapshotBranch out of null branches.
Test Plan
noticed the bug in production; the tests that perform a comparison of in-memory storage contents now only test these the data superficially and need to be improved.
Migrated from D4473 (view on Phabricator)