An operator reported migrations being very slow and sent a log with three
interleaved runs — and no way to tell one database migrated three times from
three databases migrated once. Each line now carries its target: the SQLite file
name, mysql:<database>, or baseline:memory for the in-memory pass that derives
the schema for a FRESH MySQL database (that one is fast and is not a real
database — in the report it sat between two slow runs and looked like a third).
A pass also announces how many migrations it will apply, so a run that applies
nothing is visibly distinct from one that does.