ignore: add failing test for a file mentioning the parent

When we mention "src" in src/.gitignore, we wrongly consider src/ itself
to be ignored.
ignore: don't leak rules into higher directories

A rule "src" in src/.gitignore must only match subdirectories of
src/. The current code does not include this context in the match rule
and would thus consider this rule to match the top-level src/ directory
instead of the intended src/src/.

Keep track fo the context in which the rule was defined so we can
perform a prefix match.