go.tools/vcs: allow compilation with go 1.0
go.tools/vcs: allow compilation with go 1.0

It would be nice to be able to use this package
as a dependency (or other go utilities in the
ecosystem that depend on this package) in
environments which have not (or cannot) for
whatever reason upgraded to newer versions of
golang.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/14283043

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