posix: fix split() for the case where the path is at the root of the filesystem
posixpath.split() strips '/' from the dirname *unless it is the root*. This
patch reproduces this behavior in posix.split(). The old behavior causes a
crash when creating a file at the root of the repo with localrepo.wfile()
when the repo is at the root of the filesystem.
(binary:application/octet-stream)