revlogv2: introduce a very basic docket file This is the first stone toward using a docket file in revlogv2. Right now the docket is very basic and only store the version number (which is -also- stored into the index file…) and the other files have fixed name. This new implementation break transactionally… but they are no test checking transactionally for revlogv2… So I take this as an opportunity to start small. They are no usage of revlogv2 outside of tests anyway. The docket keeps the `.i` naming used by previous version index to preserve a unique entry point. We could decide to use a different name and look it up first, or to fully rework this in a future "store" version. However that does not seems necessary right now. We will re-introduces transactionality (and associated testing…) in a later changesets. A long list of TODOs have been added to the relevant comment. Differential Revision: https://phab.mercurial-scm.org/D10624