make test-mq-qdelete work reliably (issue1180)
if after a and b has been applied, their hash starts with c, then the test
would fail (because there would be a rev c which would be seen as "applied")
so probability of failure would be 2*1/16.
# should print nothing
# should fail
abort: path 'b/x' is inside repo 'b'
abort: path 'b/x' is inside repo 'b'
# should fail
abort: path 'b/x' is inside repo 'b'
# should arguably print nothing
# should fail
abort: path 'b/a' is inside repo 'b'