Notes API: prune_notes(): Prune notes that belong to non-existing objects

Notes API: prune_notes(): Prune notes that belong to non-existing objects

When an object is made unreachable by Git, any notes that annotate that object
are not automagically made unreachable, since all notes are always trivially
reachable from a notes ref. In order to remove notes for non-existing objects,
we therefore need to add functionality for traversing the notes tree and
explicitly removing references to notes that annotate non-reachable objects.
Thus the notes objects themselves also become unreachable, and are removed
by a later garbage collect.

prune_notes() performs this traversal (by using for_each_note() internally),
and removes the notes in question from the notes tree.

Note that the effect of prune_notes() is not persistent unless a subsequent
call to write_notes_tree() is made.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>