dirstate: introduce a "tracked-key" feature A new format variant is introduced. When used, a `tracked-key` file will be generated. That file will be update when the set of tracked file might have changed. This will be useful for external automation (e.g. build tool) to be notified when the set of relevant files changes. One of the motivation for this changes is to mitigate effect dirstate-v2 has on such automation. Since the dirstate file is updated much more frequently on dirstate-v2, monitoring update to that file is no longer a viable strategy. See the associated documentation for details about the feature To prevent older client to update the repository without updating that file, a new requirements is introduced. The `postfinalizegenerators` business is a bit weird, so I'll likely clean that up soon. Differential Revision: https://phab.mercurial-scm.org/D12124