godoc: fix ToAbsolute mapping
godoc: fix ToAbsolute mapping

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 issue 1157.

R=rsc
CC=golang-dev
http://codereview.appspot.com/5278050

wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old