bin/get-pick-list.sh: tweak the commit sha matching pattern

Currently we match on:
 - any arbitrary length of,
 - any a-z A-Z and 0-9 characters

At the same time, a commit sha consists of lowercase hexadecimal
numbers. Any sha shorter than 8 characters is ambiguous - in some cases
even 11+ are required.

So change the pattern to a-f0-9 and adjust the length to 8-40.

As we're here we could use a single grep, instead of the grep/sed combo.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit 533fead4)