zsh completion: disable defaults and verbose output when completing

If ui.verbose is set to True in hgrc, commands used to provide completions
would yield bad results. For example, qpush completion would include the
patch numbers and statuses as returned by qunapplied -v, instead of just
the patch names.

Defaults are also disabled when completing to prevent similar issues when
an option is set that changes a command's output.

contrib/zsh_completion

@@ -364,7 +364,8 b' typeset -A _hg_cmd_globals'
   '--remotecmd[specify hg command to run on the remote side]:')
 
 _hg_cmd() {
-  _call_program hg hg "$_hg_cmd_globals[@]" "$@"
+  _call_program hg hg --config ui.verbose=0 --config defaults."$1"= \
+    "$_hg_cmd_globals[@]" "$@"
 }
 
 _hg_cmd_add() {