builtin/notes: fix leaking `struct notes_tree` when merging notes

builtin/notes: fix leaking `struct notes_tree` when merging notes

We allocate a `struct notes_tree` in `merge_commit()` which we then
initialize via `init_notes()`. It's not really necessary to allocate the
structure though given that we never pass ownership to the caller.
Furthermore, the allocation leads to a memory leak because despite its
name, `free_notes()` doesn't free the `notes_tree` but only clears it.

Fix this issue by converting the code to use an on-stack variable.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>