rebase: "Failed to release savepoint" when applying base2their

[wonder-sk]

Seen in the logs - in some very rare cases, when there are no "our" changes, but there are some "their" changes, rebase fails when applying base2their changeset at the last step, when committing the changes (in sqlite3 speak, releasing savepoint). In logs, this shows as Failed to release savepoint, but with no further information.

In Sqlite3SavepointTransaction, we could add more detailed error reporting, to better understand possible source of the problem.