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>

bin/get-extra-pick-list.sh

@@ -10,8 +10,7 @@
 # $ bin/get-extra-pick-list.sh | tee picklist
 
 # Use the last branchpoint as our limit for the search
-# XXX: there should be a better way for this
-latest_branchpoint=`git branch | grep \* | cut -c 3-`-branchpoint
+latest_branchpoint=`git merge-base origin/master HEAD`
 
 # Grep for commits with "cherry picked from commit" in the commit message.
 git log --reverse --grep="cherry picked from commit" $latest_branchpoint..HEAD |\