lfconvert: use a `changing_parents` context to clear the dirstate

Not sure if this is the right context, but it works and it is consistent with
the other usages of `dirstate.clear`.

hgext/largefiles/lfcommands.py

@@ -219,7 +219,8 b' def lfconvert(ui, src, dest, *pats, **op'
         success = True
     finally:
         if tolfile:
-            with rdst.dirstate.changing_files(rdst):
+            # XXX is this the right context semantically ?
+            with rdst.dirstate.changing_parents(rdst):
                 rdst.dirstate.clear()
             release(dstlock, dstwlock)
         if not success: