Change all pwd calls for $PWD variable

This avoids spawning additional processes as $PWD **always** contains
the current working directory.

plugins/svn/svn.plugin.zsh

@@ -77,7 +77,7 @@ function svn_dirty() {
 
 function svn_dirty_choose_pwd () {
   if in_svn; then
-    root=`pwd`
+    root=$PWD
     if $(svn status $root 2> /dev/null | grep -Eq '^\s*[ACDIM!?L]'); then
       # Grep exits with 0 when "One or more lines were selected", return "dirty".
       echo $1