bin/get-pick-list.sh: simplify git oneline printing

Currently we force disable the pager via "|cat" where --no-pager
exists. Additionally we could use git show instead of git log -n1.

Use those for a slightly more understandable code.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit 559c32d2)