DirFS: Handle paths with no leading / (#1638)

[metadaddy]

If you wrap an S3FileSystem with DirFileSystem, ls() breaks, since DirFileSystem assumes that the underlying FS returns paths with a leading /, but S3FileSystem does not (see #1638).

This change to _relpath() removes the leading / from the prefix used in constructing the relative path when the incoming path has no leading /, but the DirFileSystem path does.

[martindurant]

Thanks for sorting this out.
It sort of feels untidy, but I can't think of a better way to do this :)

[metadaddy]

Yeah - I spent a while looking at DirFS and S3FS before concluding that this was the way. The root cause, I think, is that fsspec itself doesn't specify whether or not a leading slash is required/expected, while DirFS assumes it will be present.

Thanks for fixing the lint nit!