git notes merge: List conflicting notes in notes merge commit message

This brings notes merge in line with regular merge's behaviour.

This patch has been improved by the following contributions:
- Ævar Arnfjörð Bjarmason: Don't use C99 comments.

Thanks-to: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/notes.c

@@ -902,7 +902,7 @@ static int merge(int argc, const char **argv, const char *prefix)
 
 	strbuf_addf(&msg, "notes: Merged notes from %s into %s",
 		    remote_ref.buf, default_notes_ref());
-	o.commit_msg = msg.buf + 7; // skip "notes: " prefix
+	strbuf_add(&(o.commit_msg), msg.buf + 7, msg.len - 7); /* skip "notes: " */
 
 	result = notes_merge(&o, t, result_sha1);