These paths use a slash for separation, and attempts to hide this behind FilePath
end up making it more complex to figure out what we should accept. Move to accept
strings and keep them as such instead of trying to convert them.

We treat a lot of the data as paths which would be available on the host
system, but this doesn't need to be the case. There are paths which the
filesystem or .NET won't accept which work just fine as paths inside
Git.

This also means the return values are also marshalled as strings, since
we can't assume they contain paths which the path marshaller would
like. It also keeps the separator as the slash rather than convert it
into something which seems native, but is different from what we'd
accept.

The paths which we do refer to as paths inside the workdir are still
marhshalled as paths.