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