notes: fix memory leak when pruning notes

In `prune_notes()` we first store the notes that are to be deleted in a
local list, and then iterate through that list to delete those notes one
by one. We never free the list though and thus leak its memory. Fix
this.

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

notes.c

@@ -1219,11 +1219,16 @@ void prune_notes(struct notes_tree *t, int flags)
 	for_each_note(t, 0, prune_notes_helper, &l);
 
 	while (l) {
+		struct note_delete_list *next;
+
 		if (flags & NOTES_PRUNE_VERBOSE)
 			printf("%s\n", hash_to_hex(l->sha1));
 		if (!(flags & NOTES_PRUNE_DRYRUN))
 			remove_note(t, l->sha1);
-		l = l->next;
+
+		next = l->next;
+		free(l);
+		l = next;
 	}
 }