Add a regression test for diff parsing with svn:mergeinfo.

* subversion/tests/libsvn_diff/parse-diff-test.c
  (unidiff_with_mergeinfo): New test diff.
  (test_parse_unidiff_with_mergeinfo, test_funcs): New test.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1600248 13f79535-47bb-0310-9956-ffa450edef68