vfs: add a `register_file` method on the vfs class This is used by the fncache vfs to register new file. Until now, `fncache` have been doing this "automatically" by monitoring write pattern. However this is fragile and when we copy files in place by other means, we need something more robuts. So we add an explicit method to do so. Differential Revision: https://phab.mercurial-scm.org/D10851

clone: use "official" API to create local clone destination This make sure we have a properly created, fully functional repository early. This will be useful to simply the hardlink/copy phases of the local clone to make it share more of its logic with the similar "stream" cloning. This has a minor impact of the test and the resulting repository has is better initialized (eg: the `wcache` directory is pre-created.) Differential Revision: https://phab.mercurial-scm.org/D10850

localrepo: introduce a clone_requirements function This function take a source repository and return a relevant set of requirements that should be used by a copy clone. This will help make the creation of the destination repository during copy clone simpler. Differential Revision: https://phab.mercurial-scm.org/D10849

createrepository: allow to directly pass the target requirements This is useful when doing a local clone that copies store contents, it will requires the destination to use the very same store requirements so directly providing them will be simpler and safer Differential Revision: https://phab.mercurial-scm.org/D10848

narrowbundle: use new context manager for silencing the ui Same reasoning as the previous change. This affects a few tests because of the hack from d7304434390f (changegroup: move message about added changes to transaction summary, 2019-09-08). Differential Revision: https://phab.mercurial-scm.org/D10886

debugbackupbundle: use new context manager for silencing the ui A difference between setting `ui.quiet` and using `ui.silent()` is that the latter also silences `ui.write()` calls. That's practically always what one wants, including here, I think. Differential Revision: https://phab.mercurial-scm.org/D10885