The implementation of splitFirst was broken;
splitFirst("foo/") must be the same as splitFirst("foo").
As a result, ToAbsolute could be simplified, and as a side
effect this fixes a long-standing bug.

Thanks to Luca Greco <luca.greco@alcacoop.it> for doing
the investigation.

Fixes #1157.

R=rsc
CC=golang-dev
https://golang.org/cl/5278050