scmutil: add option to register summary callbacks as transaction validators We have a list of summary callbacks which are run after the transaction is closed to show what has changed and what not. This patch makes it possible to register those callbacks as transaction validators so that we can show summary before committing the transaction and prompt user to accept the changes. The goal of this is to implement `pull --confirm`. Differential Revision: https://phab.mercurial-scm.org/D8199