config: add a missing preparewrite() call

Thanks Yuya for pointing this out in D808.

Differential Revision: https://phab.mercurial-scm.org/D924

mercurial/config.py

@@ -177,6 +177,7 b' class config(object):'
                 if sections and section not in sections:
                     continue
                 if self.get(section, name) is not None:
+                    self._data[section] = self._data[section].preparewrite()
                     del self._data[section][name]
                 self._unset.append((section, name))
                 continue