t6036: Test index and worktree state, not just that merge fails

t6036: Test index and worktree state, not just that merge fails

c94736a (merge-recursive: don't segfault while handling rename clashes
2009-07-30) added this testcase with an interesting corner case test,
which previously had cased git to segfault.  This test ensures that the
segfault does not return and that the merge correctly fails; just add
some checks that verify the state of the index and worktree after the merge
are correct.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>