Fix hg call when missing on archlinux with command-not-found (#5218)

themes/agnoster.zsh-theme

@@ -128,6 +128,7 @@ prompt_git() {
 }
 
 prompt_hg() {
+  (( $+commands[hg] )) || return
   local rev status
   if $(hg id >/dev/null 2>&1); then
     if $(hg prompt >/dev/null 2>&1); then