realpath: fix problems with root handling

realpath: fix problems with root handling

When --relative-base is /, all other paths should be treated as
relative (except for // where it matters).

Also, on platforms like Cygwin where / and // are distinct, realpath
was incorrectly collapsing // into /.  http://debbugs.gnu.org/10472.

* src/realpath.c (path_prefix, path_common_prefix): Treat /
and // as having no common match.
(relpath): Allow for no match even without --relative-base.
* NEWS: Document this.