When computing negative ignores, we throw away any rule which does not
undo a previous rule to optimize. But on case insensitive file systems,
we need to keep in mind that a negative ignore can also undo a previous
rule with different case, which we did not yet honor while determining
whether a rule undoes a previous one. So in the following example, we
fail to unignore the "/Case" directory:

    /case
    !/Case

Make both paths checking whether a plain- or wildcard-based rule undo a
previous rule aware of case-insensitivity. This fixes the described
issue.