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.

revert: explicitly track added but deleted file Added + deleted file are files that need to be untracked from the dirstate but that are already missing on disk. The current `_performrevert` code is handling that with exception catching. We will be able to do better with a dedicated set.

revert: have an explicit action for "forget" The distinction between "remove" and "forget" used to be in special logic checking for the state of the file in the dirstate. Now that we have dedicated filtering, we can stop relying on this logic and have two distinct actions.

revert: split between newly added file and file added in other changeset These two kinds of files are handled differently. One is deleted and the other is just forgotten (the file is untracked but left in place). The distinction is done in the `_performrevert` code itself and we would like to get ride of this.