Don't clobber chpwd

plugins/dirhistory/dirhistory.plugin.zsh

@@ -49,7 +49,9 @@ function push_future() {
 }
 
 # Called by zsh when directory changes
-function chpwd() {
+typeset -ga chpwd_functions
+chpwd_functions+='chpwd_dirhistory'
+function chpwd_dirhistory() {
   push_past $PWD
   # If DIRHISTORY_CD is not set...
   if [[ -z "${DIRHISTORY_CD+x}" ]]; then