mailer.py: provide a "repos_dir basename" substitution variable.

Patch by: Janos Gyerik <janos.gyerik@gmail.com>
(log message by me)
Review by: breser, cmpilato, danielsh

* tools/hook-scripts/mailer/mailer.py
  (main): Define 'repos_basename' for substitution.

* tools/hook-scripts/mailer/mailer.conf.example
  (SUBSTITUTIONS): Document "%(repos_basename)s".


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1439592 13f79535-47bb-0310-9956-ffa450edef68