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

pombredanne/go-deleteme,tv42/old-go,gnanderson/go,tv42/old-go,dsriram/golang,apparentlymart/golang,gnanderson/go,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,dsriram/golang,gnanderson/go,apparentlymart/golang,apparentlymart/golang,tv42/old-go,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,dsriram/golang