mercurial: separate command options and positional arguments with "--"
We don't have much problems here thanks to hgtarget(path) and CGI.escape(),
which prepends a repository path and encodes "=" character respectively, but
it's better to not rely on the side effect of these functions.
Contributed by Yuya Nishihara.
git-svn-id: http://svn.redmine.org/redmine/branches/3.4-stable@17067 e93f8b46-1217-0410-a6f0-8f06a7374b81