Fix negative ignore rules with patterns - #4296

Ignore rules allow for reverting a previously ignored rule by prefixing
it with an exclamation mark. As such, a negative rule can only override
previously ignored files. While computing all ignore patterns, we try to
use this fact to optimize away some negative rules which do not override
any previous patterns, as they won't change the outcome anyway.

In some cases, though, this optimization causes us to get the actual
ignores wrong for some files. This may happen whenever the pattern
contains a wildcard, as we are unable to reason about whether a pattern
overrides a previous pattern in a sane way. This happens for example in
the case where a gitignore file contains "*.c" and "!src/*.c", where we
wouldn't un-ignore files inside of the "src/" subdirectory.

In this case, the first solution coming to mind may be to just strip the
"src/" prefix and simply compare the basenames. While that would work
here, it would stop working as soon as the basename pattern itself is
different, like for example with "*x.c" and "!src/*.c. As such, we
settle for the easier fix of just not optimizing away rules that contain
a wildcard.

This test is by Carlos Martín Nieto.

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.