templater: simplify merge of __base__ dicts by reading it first

phase: add a dedicated pretxnclose-phase hook This new hook mirror the newly introduced 'txnclose-phase' but can abort the transaction.

phase: add a dedicated txnclose-phase hook The new 'txnclose-phase' hook expose the phase movement information stored in 'tr.changes['phases]'. To provide a simple and straightforward hook API to the users, we introduce a new hook called for each revision affected. Since a transaction can affect the phase of multiple changesets, updating the existing 'txnclose' hook to expose that information would be more complex. The data for all moves will not fit in environment variables and iterations over each move would be cumbersome. So the introduction of a new dedicated hook is preferred in this changesets. This does not exclude the addition of the full phase movement information to the existing 'txnclose' in the future to help write more complex hooks.