Dir-only pattern with a wildcard: the directory matches, nothing inside it does #23

A dir-only pattern whose last segment has a wildcard and ends in literal text matches the directory,
but nothing inside it:

.gitignore: *.egg-info/

mypkg.egg-info/              git: ignored   this library: ignored
mypkg.egg-info/PKG-INFO      git: ignored   this library: NOT ignored   <--
mypkg.egg-info/sub/x.txt     git: ignored   this library: NOT ignored   <--

build/ is fine, and so is *.egg-info without the trailing slash. What breaks it is the
fast-reject in match (gitignore.go:359, and the same line in matchDetail:376):

if p.literalSuffix != "" && !strings.HasSuffix(lastSeg, p.literalSuffix) {
    continue
}

lastSeg is the last segment of the queried path. That is a sound test when the pattern is
compared against the whole path, but a dir-only pattern can match an ancestor — PKG-INFO
doesn't end in .egg-info and never will, so the pattern is dropped before matchPattern gets to
its descendant loop. Adding && !p.dirOnly to both lines fixes it; go test ./..., gofmt -l and
go vet stay clean, and it moves nothing on my conformance bench (4,463 nested-.gitignore
queries against git check-ignore, 5 divergences before and after — 3 of those 5 are #22, the
other 2 are unrelated).

Prevalence, since the shape sounds exotic and isn't: taking the rule files of 33 large repos as
written, 9 distinct lines hit this — *.egg-info/ in airflow, transformers, langchain, vscode,
flask and pytorch, *.framework/ in tensorflow, testdata/**output/ in grafana. [Bb]uild/ has
the same shape and does not fail, which is why I measured each line instead of counting the form.

My bench missed it, for the record: it never asks about a file inside one of those directories.