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://codereview.appspot.com/14283043

Committer: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

t-yuki/go.tools.old,tmc/mirror-go.tools,t-yuki/go.tools.old,tmc/mirror-go.tools,t-yuki/go.tools.old,tmc/mirror-go.tools