dirstate: add dedicated function for updating data of a file

`dirstate.normal()` is too generic to be a user facing function for that. This
is a part of effort to refactor dirstate APIs and make them clearer.

Differential Revision: https://phab.mercurial-scm.org/D11076

mercurial/dirstate.py

@@ -593,6 +593,14 b' class dirstate(object):'
         else:
             assert False, 'unreachable'
 
+    @requires_parents_change
+    def update_parent_file_data(self, f, filedata):
+        """update the information about the content of a file
+
+        This function should be called within a `dirstate.parentchange` context.
+        """
+        self.normal(f, parentfiledata=filedata)
+
     def _addpath(
         self,
         f,