ignore: fix determining whether a shorter pattern negates another

When computing whether we need to store a negative pattern, we iterate
through all previously known patterns and check whether the negative
pattern undoes any of the previous ones. In doing so we call `wildmatch`
and check it's return for any negative error values. If there was a
negative return, we will abort and bubble up that error to the caller.

In fact, this check for negative values stems from the time where we
still used `fnmatch` instead of `wildmatch`. For `fnmatch`, negative
values indicate a "real" error, while for `wildmatch` a negative value
may be returned if the matching was prematurely aborted. A premature
abort may for example also happen if the pattern matches a prefix of the
haystack if the pattern is shorter. Returning an error in that case is
the wrong thing to do.

Fix the code to compare for equality with `WM_MATCH`, only. Negative
values returned by `wildmatch` are perfectly fine and thus should be
ignored. Add a test that verifies we do not see the error.

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