gopack: fix prefix bug
gopack: fix prefix bug

When prefix doesn't match, was deleting entire .6 file.

R=dsymonds
CC=golang-dev
http://codereview.appspot.com/4427041

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