exchange: use context manager for locks and transaction in unbundle() Note that the transactionmanager doesn't actually create a transaction -- that is done the first time .transaction() is called on it (if at all). Consequently, .close() and .release() won't do anything if no transaction has been created. This makes it a little unusual, but it still works as a context manager. Differential Revision: https://phab.mercurial-scm.org/D1841