- load_plain_commit(+Store, +Hash, -Meta:dict) is semidet
- Load the commit data as a dict. Loaded commits are cached in
commit/3. Note that only adding a fact to the cache is
synchronized. This means that during a race situation we may
load the same object multiple times from disk, but this is
harmless while a lock around the whole predicate serializes
loading different objects, which is not needed.