Fix VACUUM with an open transaction

Database.vacuum() passed VACUUM directly to SQLite, which rejects it while a transaction is open. Commit first so the maintenance operation can run, and cover the reported transaction path with a regression test.

Fixes #479

sqlite_utils/db.py

@@ -1897,6 +1897,7 @@ class Database:
 
     def vacuum(self) -> None:
         "Run a SQLite ``VACUUM`` against the database."
+        self.commit()
         self.execute("VACUUM;")
 
     def analyze(self, name: Optional[str] = None) -> None: