fix($location): rewrite relative paths correctly path==='/' in legacy html5Mode

Currently, legacy browsers get to use a clever scheme for resolving relative
URIs in html5Mode, and resolve the URI relative to $location.path().

Currently, $location.path() can be '/' under certain circumstances, which
means that when we split $location.path() on '/' and later join by '/' after
adding another path component, we end up with '//pathComponent'. $$rewrite
fails to deal with this correctly, and effectively the $location is never
changed from the root path.

This CL corrects this by ensuring that the duplicate '/' situation does not
occur when resolving relative URIs.