kernel-yocto: replace --is-ancestor with basic git porcelain commands
--is-ancestor is a relatively new git option [commit
5907cda1, Aug 30 2012].
To support build machines with older versions of git installed, we can use
the basic porcelain commands to acheive the same check.
merge-base: "--is-ancestor A B" can be replaced with:
if test "$(git rev-parse --verify A)" = "$(git merge-base A B)"
Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
