branching: merge default into stable for 6.7rc0

branching: merge stable into default

perf: add a --as-push option to perf::unbundle This turned out to make a quite significant difference.

chainsaw-update: exit early if one of the intermediate command fails That will prevent the user to be presented with a start that pretend to be consistent with the request, but is not.

chainsaw-update: lock the repository for the duration of the operation This should prevent and catch some misusage where something else try to touch the repository.

chainsaw-update: taking care of initial cloning Perhaps we should go just a bit lower level than this `instance()`, since the main added value in our use-case is full path resolution, that we need to do anyway for the rmtree cleanup.

chainsaw-update: use a graph with branching in graph This will be relevant for the next improvement of `chainsaw-update`.

chainsaw-update: log actual locks breaking Previously, the command would simply state that it was about to break locks, not if there was actually some to break. This version is race-free. It would be also possible to display the content of the lock before hand (not race-free but informative in almost all cases).

vfs: have tryunlink tell what it did It is useful in certain circumstances to know whether vfs.tryunlink() actually removed something or not, be it for logging purposes.

chainsaw: new extension for dangerous operations The first provided command is `chainsaw-update`, whose one and single job is to make sure that it will pull, update and purge the target repository, no matter what may be in the way (locks, notably), see docstring for rationale.