bin/get-typod-pick-list.sh: limit `git grep ...' to only as needed

bin/get-typod-pick-list.sh: limit `git grep ...' to only as needed

The currently used range HEAD..origin/master is far too broad. It looks
for nominations within the already_landed list (branchpoint..HEAD).

Similarly we look for already_landed whiting the [possible] nominations
Rand branchpoint..origin/master.

Improve things by limiting the look ups to the branch point.

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>