Add a regression test for issue #4155 'Merge conflict text of expanded keyword
incorrect when svn:keyword property value removed'. This test passes on
trunk and fails on 1.7.8.

* subversion/tests/cmdline/merge_tests.py
  (merge_conflict_when_keywords_removed): New test.
  (test_list): Run it.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1430504 13f79535-47bb-0310-9956-ffa450edef68