Revert "cmd/go: use shallow clones for new git checkouts"

This reverts commit bc1f9d20.

The current go-get strategy doesn't support cases that servers
cannot handle shallow clients.

Also, `go get -u` is broken and is not compatible with already
go-getted unshallow repos.

Fixes #13213.
Fixes #13206.

Change-Id: Ie89d7603d96d323db64ad82997793fda0972f709
Reviewed-on: https://go-review.googlesource.com/16832


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>