A pattern with only a trailing slash should be treated as a glob

[segevfiner]

A pattern like "bar/" should still ignore "foo/bar/", but "/bar/" should
not.

Related issue: #526

Reproduction

git init temp && cd temp
mkdir -p test/foo
cat > .gitignore <<EOF
foo/
EOF
git check-ignore -v test/foo

Output:

.gitignore:1:foo/       test/foo

(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/foo/')
# None

[jelmer]

Merged, thanks!