vfs: add "open()" for newly added code paths which open files via vfs

This patch replaces "open()" by own "__call__()" defined in derived
classes at the first invocation, for efficiency of succeeding
invocations.

mercurial/scmutil.py

@@ -217,6 +217,10 b' class abstractvfs(object):'
                 raise
         return ""
 
+    def open(self, path, mode="r", text=False, atomictemp=False):
+        self.open = self.__call__
+        return self.__call__(path, mode, text, atomictemp)
+
     def read(self, path):
         fp = self(path, 'rb')
         try: