imagebuildah: simplify screening by top layers
imagebuildah: simplify screening by top layers

We only care about the most recent top layer when we're checking if a
candidate image might be based on the one that's the base for the
current working container, so don't bother tracking more than that.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

Closes: #1646
Approved by: TomSweeneyRedHat

TomSweeneyRedHat/buildah,rhatdan/buildah,TomSweeneyRedHat/buildah,projectatomic/buildah,nalind/buildah-1,projectatomic/buildah,nalind/buildah-1,rhatdan/buildah,nalind/buildah-1,projectatomic/buildah,nalind/buildah,TomSweeneyRedHat/buildah,rhatdan/buildah,nalind/buildah