strip: introduce a soft strip option This is the first user-accessible way to use the archived phase introduced in 4.8. This implements a feature discussed during the Stockholm sprint, using the archived phase for hiding changesets. The archived phase behaves exactly as stripping: changesets are no longer visible, but pulling/unbundling them will make then reappear. The only notable difference is that unlike hard stripping, soft stripping does not affect obsmarkers. The next changeset will make use of the archived phase for history rewriting command. However, having a way to manually trigger the feature first seems a necessary step before exposing users to this phase; there is a way to un-archived changesets (unbundling), so there must be a way to archive them again. Adding a flag to strip is a good way to provide access to the feature without taking a too big risk on the final UI we want. The flag is experimental so it won't be exposed by default. Using the archived phase is faster and less traumatic for the repository than actually stripping changesets.

Our full contribution guidelines are in our wiki, please see:

https://www.mercurial-scm.org/wiki/ContributingChanges

If you just want a checklist to follow, you can go straight to

https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist

If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).