fold: replace a compat condition with a less specific proxy
If `scmutil.cleanupnodes()` is overridden, the check in
`scmutil.cleanupnodes.__code__.co_varnames` will fail. Let's use a
less specific proxy for that condition so extensions are safely
override `scmutil.cleanupnodes()`. I picked `util.safehasattr(scmutil,
'nullrev')` as the proxy. That basically checks for Mercurial commit
d739f423bf06, which is two commits later than the commit we actually
care about.