go.tools/dashboard: fix Repo creation in buildSubRepo to have valid Master repo.
go.tools/dashboard: fix Repo creation in buildSubRepo to have valid Master repo.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/13568047

maddyonline/tools,dmitris/tools,glasser/golang-tools,golang/tools,glasser/golang-tools,cmars/tools,glasser/golang-tools,josharian/tools,4ad/tools,golang/tools,jdhenke/tools,springning/tools,golang/tools,nicot/tools,josharian/tools,cw2018/tools,nicot/tools,sshetty10/tools,runcom/tools,cw2018/tools,matryer/tools,runcom/tools,dmitris/tools,josharian/tools,sshetty10/tools,sshetty10/tools,runcom/tools,glasser/golang-tools,cw2018/tools,cmars/tools,springning/tools,cw2018/tools,matryer/tools,magiccao/tools,magiccao/tools,jdhenke/tools,matryer/tools,magiccao/tools,springning/tools,argandas/tools,syreclabs/go-tools,josharian/tools,4ad/tools,argandas/tools,runcom/tools,argandas/tools,springning/tools,golang/tools,maddyonline/tools,cmars/tools,jdhenke/tools,golang/tools,nicot/tools,argandas/tools,jdhenke/tools,syreclabs/go-tools,matryer/tools,dmitris/tools,cmars/tools,maddyonline/tools,sshetty10/tools,magiccao/tools,syreclabs/go-tools,maddyonline/tools,syreclabs/go-tools,syreclabs/go-tools,dmitris/tools,nicot/tools,syreclabs/go-tools,4ad/tools