notes: extract logic into set_display_notes()

notes: extract logic into set_display_notes()

Instead of open coding the logic that tweaks the variables in
`struct display_notes_opt` within handle_revision_opt(), abstract away the
logic into set_display_notes() so that it can be reused.

Signed-off-by: Denton Liu <redacted>
Signed-off-by: Junio C Hamano <redacted>