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.

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