phases: remove excessive optimization from newheads() (issue5939) This function is intended to compute 'heads(::heads - roots::)', but it failed because 'heads + parents(roots)' missed sibling branches of the roots. That's why the public heads slipped down from D to B in the example added by 2a227782e754 "tests: add test demonstrating phase loss when cloning": > E draft > |\Z draft > | Y draft > D | public > | X draft > C/ public > B public > A public where heads = {E, Z}, roots = {X}