Correctly handle include, exclude than include in .gitignore

[segevfiner]

For example, for a .gitignore file like:

test
!test
test

"test" should be ignored.

Related issue: #526

Reproduction

git init temp && cd temp
touch test
cat > .gitignore <<EOF
test
!test
test
EOF
git check-ignore -v test

Output:

.gitignore:3:test       test

(The file is ignored)

Dulwich (before fix):

>>> from dulwich import ignore
>>> with open('.gitignore', 'r') as f:
...     a = ignore.IgnoreFilter(ignore.read_ignore_patterns(f))
...
>>> a.is_ignored('test')
False

[jelmer]

Merged, thanks!