cmd/api: update the API checker to Go 1.4 and git

Still using the ancient go/types API. Updating that to the modern API
should be a separate effort in a separate change.

Change-Id: Ic1c5ae3c13711d34fe757507ecfc00ee883810bf
Reviewed-on: https://go-review.googlesource.com/1404
Reviewed-by: David Symonds <dsymonds@golang.org>