next-prev: replace the error hint to mention about `--merge` flag

tests: add tests for conflicts while respecting commands.update.check This patch adds tests to check that we don't run into conflicts while respecting commands.update.check. The error message now does not show the hint about --merge option which we should fix.

next: respect `commands.update.check` config option in `hg next` (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 next` 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.

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.