commit: add ui.allowemptycommit config option This adds a config flag that enables a user to make empty commits. This is useful in a number of cases. For instance, automation that creates release branches via bookmarks may want to make empty commits to that release bookmark so that it can't be fast-forwarded and so it can record information about the release bookmark's creation. This is already possible with named branches, so making it possible for bookmarks makes sense. Another case we've wanted it is for mirroring repositories into Mercurial. We have automation that syncs commits into hg by running things from the command line. The ability to produce empty commits is useful for syncing unusual commits from other VCS's. In general, allowing the user to create the DAG as they see fit seems useful, and when I mentioned this in IRC more than one person piped up and said they were already hacking around this limitation by using mq, import, and commit-dummy-change-then-amend-the-content-away style solutions.