[PATCH 00 of 13] Cleanup of the purge extension

[Emanuele Aina]

I've cleaned up the purge extension, hoping in its inclusion in hgext:
 - it prints now relative pathnames
 - it uses the mercurial fs walker
   * dirstate.statwalk() now yields directories too but they are 
     filtered out in dirstate.stat() so noone else should be affected
 - it has a testcase
 - it uses the mercurial i18n module
 - it is smaller and simpler

The last patch moves it out of contrib/ putting it in hgext/ and
updates the test for the changed location.