Change command verification

Changed command verification because 'which' does sometimes produce an empty tac variable.

zsh-peco-history.zsh

@@ -8,8 +8,8 @@
 if which peco &> /dev/null; then
   function peco_select_history() {
     local tac
-    (which gtac &> /dev/null && tac="gtac") || \
-      (which tac &> /dev/null && tac="tac") || \
+    ((($+commands[gtac])) && tac="gtac") || \
+      (($+commands[tac])) && tac="tac" || \
       tac="tail -r"
     BUFFER=$(fc -l -n 1 | eval $tac | \
                peco --layout=bottom-up --query "$LBUFFER")