lib/oe/patch.py: Ignore scissors line on applying patch

The "devtool modify" could remove message body before scissors line, so
patches re-generated from git tree were incorrectly modified.
Adding --no-scissors to "git am" invocation to prevent this behaviour.

[YOCTO #12674]

Signed-off-by: Tomasz Dziendzielski <tomasz.dziendzielski@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>