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
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