notes: track whether notes_trees were changed at all

Currently, the notes copying is a bit wasteful since it always creates
new trees, even if no notes were copied at all.

Teach add_note() and remove_note() to flag the affected notes tree as
changed ('dirty').  Then teach builtin/notes.c to use this knowledge
and avoid committing trees that weren't changed.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Acked-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-notes.c

@@ -249,6 +249,8 @@ int commit_notes(struct notes_tree *t, const char *msg)
 		t = &default_notes_tree;
 	if (!t->initialized || !t->ref || !*t->ref)
 		die("Cannot commit uninitialized/unreferenced notes tree");
+	if (!t->dirty)
+		return 0; /* don't have to commit an unchanged tree */
 
 	/* Prepare commit message and reflog message */
 	strbuf_addstr(&buf, "notes: "); /* commit message starts at index 7 */

notes.c

@@ -933,6 +933,7 @@ void init_notes(struct notes_tree *t, const char *notes_ref,
 	t->ref = notes_ref ? xstrdup(notes_ref) : NULL;
 	t->combine_notes = combine_notes;
 	t->initialized = 1;
+	t->dirty = 0;
 
 	if (flags & NOTES_INIT_EMPTY || !notes_ref ||
 	    read_ref(notes_ref, object_sha1))
@@ -1011,6 +1012,7 @@ void add_note(struct notes_tree *t, const unsigned char *object_sha1,
 	if (!t)
 		t = &default_notes_tree;
 	assert(t->initialized);
+	t->dirty = 1;
 	if (!combine_notes)
 		combine_notes = t->combine_notes;
 	l = (struct leaf_node *) xmalloc(sizeof(struct leaf_node));
@@ -1026,6 +1028,7 @@ void remove_note(struct notes_tree *t, const unsigned char *object_sha1)
 	if (!t)
 		t = &default_notes_tree;
 	assert(t->initialized);
+	t->dirty = 1;
 	hashcpy(l.key_sha1, object_sha1);
 	hashclr(l.val_sha1);
 	return note_tree_remove(t, t->root, 0, &l);

notes.h

@@ -40,6 +40,7 @@ extern struct notes_tree {
 	char *ref;
 	combine_notes_fn *combine_notes;
 	int initialized;
+	int dirty;
 } default_notes_tree;
 
 /*