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
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.