selftest/devtool: Check branch in git fetch

The commit 'meta/scripts: Manual git url branch additions (dc53fe75cc)'
forget the url branch= parameter in the devtool git fetch test.

Signed-off-by: Stefan Herbrechtsmeier <stefan.herbrechtsmeier@weidmueller.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>