.arcconfig

cleanupnodes: preserve phase of parents of new nodes

As Yuya noted in the review of D3818, passing in
targetphase=phases.draft would result in advancing the phase boundary
of a secret-phase parent. We never pass targetphase=phases.draft so
far, but it's a bug waiting to happen.

I tried to refactor it so max(parentphase, X) happened in one place,
but I couldn't come up with good variables names and I ended up with a
"newphase = max(newphase, parentphase)" line, which made the whole
block not look any better to me.

Differential Revision: https://phab.mercurial-scm.org/D3824

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