dirstate: use `dirstate.change_files` to scope the change in `lfconvert`

This is the way.

hgext/largefiles/lfcommands.py

@@ -219,6 +219,7 b' def lfconvert(ui, src, dest, *pats, **op'
         success = True
     finally:
         if tolfile:
+            with rdst.dirstate.changing_files(rdst):
             rdst.dirstate.clear()
             release(dstlock, dstwlock)
         if not success: