go.tools/dashboard/builder: make old commit watcher opt-in and exclusive
go.tools/dashboard/builder: make old commit watcher opt-in and exclusive

Only gccgo should be using this now, and hopefully not for too long.
Turn it off by default, and make it the only thing that happens
when it's switched on.

LGTM=bradfitz, cmang
R=bradfitz, cmang
CC=golang-codereviews
https://golang.org/cl/153940043

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