Use SUDO_COMMAND to check for sudo

`sudo -n true` only checks that we _could_ use `sudo`, not if we are in
a sudo session.

closes #852

powerlevel9k.zsh-theme

@@ -623,12 +623,12 @@ prompt_context() {
   if [[ $(print -P "%#") == '#' ]]; then
     current_state="ROOT"
   elif [[ -n "$SSH_CLIENT" || -n "$SSH_TTY" ]]; then
-    if sudo -n true 2>/dev/null; then
+    if [[ -n "$SUDO_COMMAND" ]]; then
       current_state="REMOTE_SUDO"
     else
       current_state="REMOTE"
     fi
-  elif sudo -n true 2>/dev/null; then
+  elif [[ -n "$SUDO_COMMAND" ]]; then
     current_state="SUDO"
   fi