cmd/godoc: redirect /x to subrepo page
cmd/godoc: redirect /x to subrepo page

Fixes golang/go#9061.

Change-Id: Ibe5764932bb8e223e7f838415f84bc330eb5d818
Reviewed-on: https://go-review.googlesource.com/12082
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

jdhenke/tools,magiccao/tools,josharian/tools,magiccao/tools,golang/tools,cw2018/tools,syreclabs/go-tools,matryer/tools,sshetty10/tools,golang/tools,glasser/golang-tools,josharian/tools,josharian/tools,nicot/tools,magiccao/tools,argandas/tools,glasser/golang-tools,argandas/tools,springning/tools,cmars/tools,syreclabs/go-tools,cmars/tools,springning/tools,runcom/tools,jdhenke/tools,sshetty10/tools,nicot/tools,runcom/tools,runcom/tools,matryer/tools,golang/tools,matryer/tools,cw2018/tools,golang/tools,sshetty10/tools,nicot/tools,cmars/tools,springning/tools,syreclabs/go-tools,runcom/tools,josharian/tools,sshetty10/tools,glasser/golang-tools,argandas/tools,magiccao/tools,jdhenke/tools,argandas/tools,cw2018/tools,golang/tools,springning/tools,jdhenke/tools,syreclabs/go-tools,cmars/tools,nicot/tools,cw2018/tools,matryer/tools,syreclabs/go-tools,syreclabs/go-tools,glasser/golang-tools