tests: better determinism in test-chg.t
chg tests fail pretty often with "Sample count: *" line disappearing.
It disappears because the sample count is zero, in which case a custom message is printed.
This commit makes the test succeed in that case.
Differential Revision: https://phab.mercurial-scm.org/D11716
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.