range-diff: output `## Notes ##` header

range-diff: output `## Notes ##` header

When notes were included in the output of range-diff, they were just
mashed together with the rest of the commit message. As a result, users
wouldn't be able to clearly distinguish where the commit message ended
and where the notes started.

Output a `## Notes ##` header when notes are detected so that notes can
be compared more clearly.

Note that we handle case of `Notes (<ref>): -> ## Notes (<ref>) ##` with
this code as well. We can't test this in this patch, however, since
there is currently no way to pass along different notes refs to `git
log`. This will be fixed in a future patch.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>