copies: consider nullrev a common ancestor I've seen many bugs in the git codebase that were caused by it not having a null revision and being forced to treat root commits differently. Mercurial has a null revision and I think it's generally a bug to treat it differently from other commits in graph algorithms. This effectively undoes 83cfa1baf8ad (copies: don't report copies with unrelated branch, 2010-01-01). The test cases that that commit added still passes. I suspect some other fix after that commit made it unnecessary. Differential Revision: https://phab.mercurial-scm.org/D5594