context: use `dirstate.set_tracked` in context.copy

This is the new shiny API.

Differential Revision: https://phab.mercurial-scm.org/D11015

mercurial/context.py

@@ -1738,10 +1738,7 b' class workingctx(committablectx):'
         else:
             with self._repo.wlock():
                 ds = self._repo.dirstate
-                if ds[dest] in b'?':
-                    ds.add(dest)
-                elif ds[dest] in b'r':
-                    ds.normallookup(dest)
+                ds.set_tracked(dest)
                 ds.copy(source, dest)
 
     def match(