It's useful to have to avoid certain conversions to happen otherwise.

…ly to git.

Previously directory excludes like `dir/` could (possibly) yield different results compared to git.
This is an opportunitstic change as it wasn't possible to trigger the wanted behaviour in a test
related to directory-specific matching. It did trigger, however, when matching normal patterns
which indicated there was indeed a bug.

Also, improve method documentation.

Otherwise it's not possible to keep allocations for matches,
while also adjusting pattern lists due to stack changes.

On the bright side, this adds some more deduplication to the implementation,
at the cost of dealing with some hashmaps that are updated when matching.