real_path(): properly handle nonexistent top-level paths

The change has two points:

1. Do not strip off a leading slash, because that erroneously turns an
absolute path into a relative path.

2. Do not remove slashes from groups of multiple slashes; instead let
chdir() handle them.  It could be, for example, that it wants to
leave leading double-slashes alone.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>