So far the most confusing aspect of the flattened single replies is when
there are sibling comments on the same level. For example, if a comment
has 2 replies (A and B) and A has one reply (C), the flattening would
put all of A, B, and C on the same indentation level as A C B. This was
confusing and made it hard to recognize where a subtree ended and it
went back to sibling comments.
This change makes it so that the flattening will only happen when there
are no siblings. This will reduce how often the flattening is applicable
somewhat, but also eliminates this confusing case entirely.