go.tools/dashboard/app: update commit in transaction on perf regression
go.tools/dashboard/app: update commit in transaction on perf regression

The sendPerfFailMail function populated a dummy commit value and
then calls commonNotify, which then updated and stored that dummy
commit, hosing the original commit entity.

LGTM=rsc
R=rsc, bradfitz, dvyukov
CC=golang-codereviews
https://golang.org/cl/164960043

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