dirstate: rename `update_file_reference` to `update_file_p1`

This is a bit clearer on the intended usage of the function. (but still not great)

Differential Revision: https://phab.mercurial-scm.org/D11135

mercurial/dirstate.py

@@ -500,7 +500,7 b' class dirstate(object):'
             return True
 
     @requires_parents_change
-    def update_file_reference(
+    def update_file_p1(
         self,
         filename,
         p1_tracked,

mercurial/scmutil.py

@@ -1487,13 +1487,13 b' def movedirstate(repo, newctx, match=Non'
     s = newctx.status(oldctx, match=match)
 
     for f in s.modified:
-        ds.update_file_reference(f, p1_tracked=True)
+        ds.update_file_p1(f, p1_tracked=True)
 
     for f in s.added:
-        ds.update_file_reference(f, p1_tracked=False)
+        ds.update_file_p1(f, p1_tracked=False)
 
     for f in s.removed:
-        ds.update_file_reference(f, p1_tracked=True)
+        ds.update_file_p1(f, p1_tracked=True)
 
     # Merge old parent and old working dir copies
     oldcopies = copiesmod.pathcopies(newctx, oldctx, match)