Pathological-merge-test-cases is a project mainly written in C++ and JAVA, it's free.
Bill Ritcher's set of 3-way merge torture tests.
This is a simple repository set up to put all of the test cases from Bill Ritcher's paper for Guiffy describing safe 3-way merge algorithms in one place, with all of the test cases sorted into directories. Using it to test a merge tool is easy:
git merge origin/other
git mergetool
All copyrights on the source files themselves are clearly marked, and in most cases belong to Bill Ritcher or Guiffy.
I'd love pointers to more examples of pathological merging, the crazier the better.
NOTE: The existence of this repository should in no way be construed as an endorsement of Guiffy, although, unsurprisingly, it does the best job of the currently available merge tools in handling the various challenges the test cases throw at it.