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>

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