histedit: allow configuring default behavior Adds a configuration setting for allowing users to specify the default behavior of 'hg histedit' without arguments. This saves users from having to manually figure out the bottom commit or a complicated revset. My current revset of choice is "only(.) & draft() - ::merge()" The commits that histedit can work with is usually quite limited, so if this feature ends up working well, we may want to consider making "only(.) & draft() - ::merge()" the default behavior for everyone.