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.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.