evolve: modify and move the comment to appropriate position

next: update the command description Now, if necessary next command evolve the next child revision by defualt. You don't need to pass the --evolve flag.

next: make next command --evolve by default Before this patch, if we need to evolve to update to the next child, we were suggesting the user to use --evolve flag. This patch make some changes to evolve by default in that conditions. After making next command to evolve by default we have to consider the following points: 1) If we don't need to evolve while updating to the next child: a) And if wdir is dirty, we suggest to use --merge flag b) if wdir is clean, we simply update to next child (if ambiguous, prompt the user to select one) 2) If we need to evolve: a) when wdir is dirty, we suggest the user to use `hg shelve` first, to make wdir clean. As we don't support --merge while evovling. b) when wdir is clean, we evolve the next cset. Changes made in test-prev-next.t reflect the changed behaviour.