doc: add the tool to check section marks in help documents This patch adds "doc/check-seclevel.py" which checks below in help documents: - whether unknown or unavailable section marks are used or not - whether appropriate section mark is used at sub-sectioning It should be invoked in "doc" directory. It checks all help documents of Mercurial (topics, commands, extensions), if no file is specified by --file option. With --file option, it checks contents of the specified file as help document, for self testing purpose: -t/-c/-e/-C are used to specify what kind of help document contents of the specified file is. This checking is related to changeset 979b107eaea2.

histedit: replaces patching logic by merges The old and fragile patching logic is replaced by smart merges (as rebase and graft do). This should prevents some conflicts and smoother human resolution. For this purpose the "foldchanges" function is renamed to "applychanges" and handle a single revision only.

histedit-test: ensure that non commute test will never commute The previous version would commute if using merge algorithm (to be accurate, merge will cleanly prompt the user during the merge). The new version create and initial commit with some content for all involved files en ensure all changes are a content changes of the first lines. This lead to guaranteed conflict when commuted.