bin/get-pick-list.sh: flesh out is_sha_nomination

Refactor is_fixes_nomination into a is_sha_nomination helper. This way
we can reuse it for more than the usual "Fixes:" tag.

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 b7418d1f)