Check if git is available, exit if not

Property definitions

nitc :: nitpm $ Sys :: check_git
# Check if `git` is available, exit if not
private fun check_git
do
	var proc = new ProcessReader("git", "--version")
	proc.wait
	proc.close

	if proc.status != 0 then
		print_error "Please install `git`"
		exit 1
	end
end
src/nitpm.nit:412,1--423,3