bin/get-extra-pick-list: use git merge-base to get the branchpoint

Since mesa development history is linear and the only diversion is at
the branchpoint. Thus we can drop the ad-hoc parsing and use git
merge-base to retrieve it.

Cc: "13.0 17.0" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
(cherry picked from commit cb1947eac70f63d78835a5442017b1f3b7099d77)
1 file changed