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

mercurial/store.py

@@ -730,6 +730,11 b' class _fncachevfs(vfsmod.proxyvfs):'
         else:
             return self.vfs.join(path)
 
+    def register_file(self, path):
+        """generic hook point to lets fncache steer its stew"""
+        if path.startswith(b'data/') or path.startswith(b'meta/'):
+            self.fncache.add(path)
+
 
 class fncachestore(basicstore):
     def __init__(self, path, vfstype, dotencode):

mercurial/vfs.py

@@ -329,6 +329,9 b' class abstractvfs(object):'
                     None  # pytype: disable=attribute-error
                 )
 
+    def register_file(self, path):
+        """generic hook point to lets fncache steer its stew"""
+
 
 class vfs(abstractvfs):
     """Operate files relative to a base directory