podman rmi: overzealous removal of referenced images

[edsantiago]

This seems to be a variant of #992. Build an image:

# podman run --name c1 fedora true
# podman commit c1 img1
# podman rm c1

Build a second image based on the first:

# podman run --name c2 img1 true
# podman commit c2 img2
# podman rm c2

Run podman images, see img1 and img2 as expected. Delete the new (second) image, and the first one is gone too:

# podman rmi img2
# podman images
<where did img1 go?>

podman-0.7.3-1.git0791210.fc28

[vrothberg]

Fixed with #1174.