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)

commit: catch changed exec bit on files from p1 (issue4382)

revert: add a `drop` action This prevents the need for a try except in the `_performrevert` code.