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
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}