The issue occurred when replying to a comment consisting of multiple
list items, starting with a <dt> (instead of the expected <dd>), so
that the comment is considered to be unindented.
Modifier tried to add the reply directly inside the list (<dl>) rather
than inside the last list item (<dt>), which caused it to be confused
about indentation levels and try to un-indent more times than there
were indentations.
The simplest solution, given the existing code, is to add the reply
outside the list instead, in a new list. This results in a "list gap"
(<dl><dt>...</dt><dd>...</dd></dl><dl><dd>...</dd></dl>), but I think
it's acceptable for this rare case.
There are separate tests cases for old Parser and for Parsoid HTML,
because they parse the original wikitext differently (with the old
Parser producing HTML with a list gap too).
Bug: T279445
Change-Id: Ie0ee960e7090cf051ee547b480c980e9530eda51