facility to ensure additive-only file operations during backups

There is a race condition for folk backing up bzr repositories via rsync
etc; we should allow a hook which would tell bzr to never move things to
obsolete_packs (implies disabling upgrade and reconcile commands too). A
plugin that hooked this might e.g. check for a /var/run file or some
such to see if it is safe to obsolete storage files.

 affects bzr

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