This test simulates a situation where the last write to the db is short.
Aggregating the changeset after reopening the file should return an
error (which includes a partially-aggregated changeset) containing an
aggregation of changesets that were fully written.
At this point, the test re-writes the final changeset (and this time it
successfully writes in full).
The file should be recoverable with all changesets, including the last
one.
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