phases: fix performance regression with Python 2
Unlike Python 3, xrange doesn't support efficient "in" and uses a linear
time scan instead. Expand the condition to handle it fast.
Differential Revision: https://phab.mercurial-scm.org/D9072
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL