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

contrib/zsh_completion

@@ -1,4 +1,4 b''
-#compdef hg
+#compdef hg chg
 
 # Zsh completion script for mercurial.  Rename this file to _hg and copy
 # it into your zsh function path (/usr/share/zsh/site-functions for