Notes API: remove_note(): Remove note objects from the notes tree structure

This includes adding internal functions for maintaining a healthy notes tree
structure after removing individual notes.

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

notes.h

diff --git a/notes.h b/notes.h
index 5f22852..9e66855 100644
--- a/notes.h
+++ b/notes.h
@@ -25,6 +25,9 @@
 void add_note(const unsigned char *object_sha1,
 		const unsigned char *note_sha1);
 
+/* Remove the given note object from the internal notes tree structure */
+void remove_note(const unsigned char *object_sha1);
+
 /* Free (and de-initialize) the internal notes tree structure */
 void free_notes(void);