This new option provides essential new functionality, changing diff
output to first parent only, without changing history traversal mode,
so it deserves its own test.
As we do it, add additional test that --diff-merges=first-parent by
itself doesn't imply -p and only outputs diffs for merge commits.
Signed-off-by: Sergey Organov <sorganov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
