run-tests: pass --with-hg to run-tests.py command used by bisect
This makes `run-tests.py -l test-run-tests.t` 23 seconds faster on my
laptop. Inside the test, `$ rt --known-good-rev=0 test-bisect.t` took 24.9
seconds before, and 1.2 seconds after.
Differential Revision: https://phab.mercurial-scm.org/D576