Before this patch, during a rebase if you get a point where you can't
figure out what to do next then either you had to complete your series
or abandon all the work you have done during this rebase.
Now, with this feature you can stop at any point by keeping the rebased
csets and mark original csets as obsolete. And if you don't have evolution
extension enabled then you can use --keep option as an alternative which
will keep original csets too, instead of marking them obsolete.
nit: 'cannot stop in --collapse session'
Also, if we cannot stop, we should not fallback to aborting.