merge with stable

commands: deprecate the parents commands It's replaced by 'hg summary' or hg log -r 'parents(foo)' and doesn't need to take up space in our command list anymore.

revset: remove nullrev from the bookmark computation Same as for other revset we sanitize the content of the set to be able to rely on it more.

revset: unify code flow in `bookmark` We refactor the code of the bookmark revset to have a single return. This will allow us to sanitize the content of the set.

revset: remove invalid value in the origin set Same as the parents related revsets, origin had some invalid value in the computed set. We remove them.

revset: remove nullrev from set computed in parents() The old code relied on the subset contents to get rid of invalid values. We would like to be able to rely more on the computation in parents() so we filter out the invalid value.

revset: refactor parents() into a single return point Both paths are doing similar thing in the end. We refactor the function so that the `ps` set is commonly used at the end. This will end excluding `nullrev` from this set in a future patch

revset: remove nullrev from set computed in p1() and p2() The old code relied on the subset contents to get rid of invalid values. We would like to be able to rely more on the computation in p1() and p2() so we filter out the invalid value

revset: document the choice made in __generatorset.__iter__ The method code looks a bit ugly but has good reasons to. We document them to prevent naive refactoring in the future.

help: mention mode in hg log --removed help (issue4381)