evolve: use --all by default (BC)
allopt defaults to None to avoid touching _checkevolveopts(). If it were True
by default, that function would always act like the flag was provided by user,
and so using --abort, --stop, --continue or --rev on their own would never work
(hg evolve will complain that you cannot specify both --rev and --all).
A notable change in tests is "no troubled changesets" line that gets printed
when using `hg evolve` to update to the evolved successor of working directory
parent, but there were no actual instabilities.