filepath: Abs must always return a clean path

filepath: Abs must always return a clean path

When I was first coding Abs, I wondered if people wouldn't
expect the path to be consistently clean, even if the path
passed in was already absolute.

CL 4524078 has a potential problem based on exactly that
assumption, so it feels like this behavior is indeed the
most useful and least surprising.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/4548074