changegroup: use the 'postclose' API on transaction The post-transaction hooks run after the lock release (because hooks may want to touch the repository), but they must only run if the transaction is successfully closed. We use the new 'addpostclose' method on transaction to register a callback installing this post-lock-release call.

transaction: allow registering a post-close callback The addchangegroup code considers the transaction done after a 'tr.close()' call and schedules the hook's execution for after lock release. In the nested transaction case, the transaction is not yet committed and we must delay this scheduling. We add an 'addpostclose' method (like the 'addpending' and 'addfinalize' ones) that registers code to be run if the transaction is successfully committed.

exchange: swap "push" for "pull" in pulloperation docstring