from-disk-exclude: handle top_path with a final slash

Before this patch giving `from_disk` and directory path with final '/'s
would confuses the exclusion code as it would assume it need to exclude
one extra character to get the basename. This would swallow the first
char of the directory to exclude, leading to a righful crash.

This would affect directory excluded in the first directory only as sub
directory would be called without a '/'.

We fix the path at the top of the function as trailing '/'s could
confuse other code too in the future.