8140449: (fs) Paths.get("x").relativize("") return ..\ on Windows

[Alan Bateman]

This issue relates to two corner cases on Windows that result in a 
leading slash. One is Paths.get("C:").toAbsolutePath(), the other is 
Paths.get("x").relativize(""). The changes are simple and I've expanded 
the test coverage to cover more cases. There is further work requires on 
relativize but that is for another issue. The proposed changes are here:

http://cr.openjdk.java.net/~alanb/8140449/webrev/

Thanks,

Alan