We're talking about 9dcf5998935c8aaa846d7585b81f0dcfe1935b3d from Sun
Mar 20 13:19:48 2016 +0000, still?
The idea is that in a (parse-partial-sexp from to), the end position
might be in the middle of a two character comment marker, such as "/*".
Before this change, it was impossible successfully to use the result of
that operation as the old state for continuing parse-partial-sexp from
that position, since it did not contain enough info to see it was in a
comment after passing the "*"
The change 9dcf599 added an extra element onto the parse state which was
non-nil when we end up after a "/", etc.