t6044 has lots of tests for verifying that merge will abort as expected
when there are changes staged before the merge starts. However, it only
checked for non-zero exit code, which could mean that the merge ran to
completion with conflicts. Check that the merge was actually correctly
aborted, i.e. that .git/MERGE_HEAD is not present.
This changes one of the tests from expect_success to expect_failure.
Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>