In several tests in merge_tests.py, change directory to the WC first, so the
paths printed during the test run are much shorter than they would otherwise
be, to ease debugging.

* subversion/tests/cmdline/merge_tests.py
  (...several tests...): As above.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1459410 13f79535-47bb-0310-9956-ffa450edef68