zsh: enable completion support for chg as well
When verifying this change, you may need to clear/rebuild the completion cache;
I did this by deleting the ~/.zcompdump file and then starting a new shell.
Differential Revision: https://phab.mercurial-scm.org/D6574
#require docutils gettext
$ $TESTDIR/check-gendoc zh_TW
checking for parse errors