prev: respect `commands.update.check` config option in `hg prev` (issue5808) `commands.update.check` config option determines what level of checking should be done when we update to another revision. When we set this config option to `noconflict`, it updates your working directory to a specified revision only when there will be no conflicts which is a very nice behavior. This patch adds logic to respect the config option so that we can have that nice behavior with `hg prev` too. It turns out, we just needed to use hg.updatetotally() and add some cases to check config option. Right now, we only respect the config if value is set to `noconflict`. Otherwise we fallback to the default behavior. This is a part of fixing issue5808. Test changes demonstrate the fix.

# name space package to host third party extensions
from __future__ import absolute_import
import pkgutil
__path__ = pkgutil.extend_path(__path__, __name__)