Fix edge case of children for images that are FROM a SharedTag

[tianon]

Before:

$ bashbrew children --uniq adoptopenjdk | wc -l
4
$ bashbrew children --uniq adoptopenjdk | cut -d: -f1 | sort -u
cassandra

After:

$ bashbrew children --uniq adoptopenjdk | wc -l
82
$ bashbrew children --uniq adoptopenjdk | cut -d: -f1 | sort -u
cassandra
gradle
groovy
maven
open-liberty
tomcat
websphere-liberty

[tianon]

(Probably best to review without whitespace, ala https://github.com/docker-library/bashbrew/pull/32/files?w=1)