reachableroots: sort the smartset in the pure version too
Changeset be8a4e0800d8 uses smartset lazy sorting for the C version. We need to
apply the same to the pure version for consistency. This is fixing the tests
with --pure.
To run the tests, do:cd tests/python run-tests.pySee http://mercurial.selenic.com/wiki/WritingTests formore information on writing tests.