Teach builtin-notes to remove empty notes

When the result of editing a note is an empty string, the associated note
entry should be deleted from the notes tree.

This allows deleting notes by invoking either "git notes -m ''" or
"git notes -F /dev/null".

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

builtin-notes.c

@@ -113,7 +113,7 @@ static void create_note(const unsigned char *object,
 
 	stripspace(buf, 1);
 
-	if (!skip_editor && !buf->len) {
+	if (!buf->len) {
 		fprintf(stderr, "Removing note for object %s\n",
 			sha1_to_hex(object));
 		hashclr(result);
@@ -197,7 +197,8 @@ int cmd_notes(int argc, const char **argv, const char *prefix)
 	struct notes_tree *t;
 	unsigned char object[20], new_note[20];
 	const unsigned char *note;
-	const char *object_ref;
+	const char *object_ref, *logmsg;
+
 	int edit = 0, show = 0;
 	const char *msgfile = NULL;
 	struct msg_arg msg = { 0, STRBUF_INIT };
@@ -271,8 +272,14 @@ int cmd_notes(int argc, const char **argv, const char *prefix)
 	}
 
 	create_note(object, &buf, msg.given || msgfile, note, new_note);
-	add_note(t, object, new_note, combine_notes_overwrite);
-	commit_notes(t, "Note added by 'git notes edit'");
+	if (is_null_sha1(new_note)) {
+		remove_note(t, object);
+		logmsg = "Note removed by 'git notes edit'";
+	} else {
+		add_note(t, object, new_note, combine_notes_overwrite);
+		logmsg = "Note added by 'git notes edit'";
+	}
+	commit_notes(t, logmsg);
 
 	free_notes(t);
 	strbuf_release(&buf);

t/t3301-notes.sh

@@ -234,6 +234,37 @@ test_expect_success 'show -m notes' '
 	test_cmp expect-m output
 '
 
+test_expect_success 'remove note with -F /dev/null (setup)' '
+	git notes edit -F /dev/null
+'
+
+cat > expect-rm-F << EOF
+commit bd1753200303d0a0344be813e504253b3d98e74d
+Author: A U Thor <author@example.com>
+Date:   Thu Apr 7 15:17:13 2005 -0700
+
+    5th
+EOF
+
+printf "\n" >> expect-rm-F
+cat expect-F >> expect-rm-F
+
+test_expect_success 'verify note removal with -F /dev/null' '
+	git log -4 > output &&
+	test_cmp expect-rm-F output &&
+	! git notes show
+'
+
+test_expect_success 'do not create empty note with -m "" (setup)' '
+	git notes edit -m ""
+'
+
+test_expect_success 'verify non-creation of note with -m ""' '
+	git log -4 > output &&
+	test_cmp expect-rm-F output &&
+	! git notes show
+'
+
 test_expect_success 'create other note on a different notes ref (setup)' '
 	: > a6 &&
 	git add a6 &&