discovery: avoid wrong detection of multiple branch heads (issue6256) This fix the code using obsolescence markers to remove "to be obsoleted" heads during the detection of new head creation from push. The code turned out to not use the branch information at all. This lead changeset from different branch to be detected as new head on unrelated branch. The code fix is actually quite simple. New tests have been added to covers these cases. Differential Revision: https://phab.mercurial-scm.org/D8259