This fixes an issue brought up by djasper@ in his review of D44790. We
handled top-level child lines, but if those child lines themselves
had child lines, we didn't handle them.
Rather than use recursion (which could blow out the stack), I use a
DenseSet to hold the set of lines we haven't yet checked (since order
doesn't matter), and update the set to add the children of each
line as we check it.
Test Plan: New tests added. Confirmed tests failed before fix
and passed after fix.