Only use the previous (base) snapshot as reference for incremental loads
The old behavior would find any object with a matching sha1_git from the archive to use as a reference point in the generated snapshot. While there's no known sha1_git collisions in the wild, this behavior is inconsistent with other loaders.
This removes the only reason to keep the object_find_by_sha1_git
api in storage.
Test Plan
tox tests unchanged
Migrated from D3181 (view on Phabricator)