Documentation improvements for the blackbox merge test.
Documentation improvements for the blackbox merge test.

* subversion/tests/cmdline/merge_tests.py
  (setup_dir_replace): Improve doc string and inline comments.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion