bin/get-pick-list.sh: handle reverts prior to the branchpoint

bin/get-pick-list.sh: handle reverts prior to the branchpoint

Currently we detect when a breaking commit:
 - has landed in stable, and
 - is referenced by a untagged fix in master

Yet we did not consider the case of breaking commit:
 - prior to the branchpoint, and
 - is referenced by a untagged fix in master

Addressing the latter is extremely slow, due to the size of the lookup.

That said, we can trivially use the existing is_sha_nomination() helper
to catch reverts.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Juan A. Suarez <jasuarez@igalia.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>