Using git merge-base for checking for ancestors is nice but required git 1.8.0
which is not in many distrbutions yet. We therefore revert to a more ugly
check using git branch --contains until such times as we can upgrade.
(Bitbake rev: 31467c0afe0346502fcd18bd376f23ea76a27d61)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>