revision: fix leaking display notes

We never free the display notes options embedded into `struct revision`. Implement a new function `release_display_notes()` that we can call in `release_revisions()` to fix this. There is another gotcha here though: we play some games with the string list used to track extra notes refs, where we sometimes set the bit that indicates that strings should be strdup'd and sometimes unset it. This dance is done to avoid a copy of an already-allocated string when we call `enable_ref_display_notes()`. But this dance is rather pointless as we can instead call `string_list_append_nodup()` to transfer ownership of the allocated string to the list. Refactor the code to do so and drop the `strdup_strings` dance. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>