merge: remove emptyactions() and use collections.defaultdict(list) instead emptyactions() used to return a dict which was populated and passed into applyupdates(). However, with recent changes, we no longer pass a plain dict, instead we pass the mergeresult object. There was only one usage of emptyactions and that too inside mergeresult object. That usage is replaced with collections.defaultdict(list) instead. Not sure why we were not using collections.defaultdict(list) from the beginning. Differential Revision: https://phab.mercurial-scm.org/D8903