check-code: forbid mutable value for default argument
default value are common to all call. Using mutable value is a classical source
of bug in Python. We forbid it.
The regexp (Courtesy of Matt Mackall) is only catching such value on the first
line of a definition, but that will be good enough for now.
Run Mercurial tests with Vagrant:
$ vagrant up
$ vagrant ssh -c ./run-tests.sh