test-chg: run only with chg
It doesn't make sense to run test-chg.t without chg, so ignore it with vanilla
hg, and specify chg executable explicitly.
test-chg.t can host chg-specific tests.
distate: add assertions to backup functions
Those assertions will prevent the backup functions from overwriting
the dirstate file in case both: suffix and prefix are empty.
(foozy suggested making that change and I agree with him)