zsh_completion: drop -w, add -S to _arguments
_arguments -w: "allow option stacking even if one or more of the options take
arguments", so something like "-xy xarg yarg". Mercurial's command line parser
doesn't support that, so the flag got removed in zsh 92584634d3d3 and
_arguments -S: "do not complete options after a ‘--’ appearing on the line". By
options it means things like "-x" and "--foo", but it will still complete
arguments like files.
Differential Revision: https://phab.mercurial-scm.org/D4262