Tests for case sensitive conflicts in the index

So after looking at https://github.com/libgit2/libgit2sharp/pull/344 , I wanted to add a test case that had conflicts in the index that differed only in case.

core git will allow a merge of two branches that contain items differ in case, regardless of your platform or your `core.ignorecase` settings and we should, I would think, be able to handle those conflicts.

My thought was that these tests would simply pass against the current code base, but to my surprise, they do not...!  I have not yet been able to debug this, but it appears that `ignorecase` is a deeper setting than I had imagined.

I did want to push up this test, though, to illustrate a problem that can arise and start some discussion.

/cc @nulltoken
/cc @dahlbyk