x/tools/go/vcs: support go.googlecode.com paths
x/tools/go/vcs: support go.googlecode.com paths

Also fix a bug in CreateAtRev, which was doing something hg-specific.

LGTM=cmang
R=cmang
CC=golang-codereviews
https://codereview.appspot.com/180540043

karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools