histedit: fix serializing of None backupfile If the histedit backupfile was None (like if evolve is enabled) it would get serialized as 'None' into the state file. Later if the histedit was aborted and the top most commit was unreachable (ex: if it was obsolete or stripped), histedit would try to unbundle the backupfile and try to read .hg/None. This fixes it to not serialize None. Since it only happens with evolve, I'm not sure how to add test coverage here.