Bug#919924: mercurial: autopkgtest depends on monotone which is not in testing

[Graham Inggs]

To be clear, dropping the test dependency on monotone does result in the 
following test being skipped:

test-convert-mtn.t
test-convert-mtn.t ... # Test test-convert-mtn.t
# Running sh "/tmp/hgtests.8zWoOH/child253/test-convert-mtn.t.sh"
skipped missing feature: monotone client (>= 1.0)
# Ret was: 80 (test-convert-mtn.t)