filectx: move size to basefilectx

See previous patch for context - avoid code duplication.

mercurial/context.py

@@ -801,6 +801,8 b' class basefilectx(object):'
         return self._copied
     def repo(self):
         return self._repo
+    def size(self):
+        return len(self.data())
 
     def path(self):
         return self._path
@@ -2065,8 +2067,6 b' class memfilectx(committablefilectx):'
 
     def data(self):
         return self._data
-    def size(self):
-        return len(self.data())
 
     def remove(self, ignoremissing=False):
         """wraps unlink for a repo's working directory"""