tests: fix isinstance test of wrong variable 3086a8627b2970cd7fbf49bc69413c08c68d5514 changed self._case to be a list, but it was forgotten to adjust this line.

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