bin/get-pick-list.sh: warn when commit lists invalid sha

We had cases where people would list old/invalid sha in the commit.
Add a trivial checker to catch those and throw a warning.

CC: Juan A. Suarez <jasuarez@igalia.com>
CC: Dylan Baker <dylan@pnwbakers.com>
CC: mesa-stable@lists.freedesktop.org
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Andres Gomez <agomez@igalia.com>
(cherry picked from commit e0dbfc99)