notes: mark unused callback parameters

notes: mark unused callback parameters

for_each_note() requires a callback, but not all callbacks need all of
the parameters. Likewise, init_notes() takes a callback to implement the
"combine" strategy, but the "ignore" variant obviously doesn't look at
its arguments at all. Mark unused parameters as appropriate to silence
compiler warnings.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>