overlaychangectx: return nullrev if commit has no parents
In Mercurial, every commit has at least one parent -- root commits have the null revision as their parent. In Git, root commits don't have any parents. This difference needs to be papered over in hg-git for 'hg incoming' to work in Mercurial 3.4+. Note that this doesn't fix all the broken tests in default -- I haven't had time to investigate the others.
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