The purpose of ensuring 'incremental fetch' is not easy to see from
the codes. So add comments to explain this.
(Bitbake rev: 8b890b87e30cd05ec92ed71ee3691a47b4d77253)
Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>