run-tests: allow bisecting a different repo Add `--bisect-repo` flag which accepts a different repo to bisect. 3rd party extensions may reuse `run-tests.py` from core to run tests. Test failure could be caused by either a core hg change or the 3rd party extension code itself. Having a way to specify which repo to bisect is useful. Differential Revision: https://phab.mercurial-scm.org/D578