cmd/go: use shallow clones for new git checkouts

Currently go get will clone the full history of git repos.
We can improve the download waiting time/size by passing depth argument.

The docs about shallow clones and the --depth argument are here:
https://git-scm.com/docs/git-clone
https://git-scm.com/docs/git-pull

Fixes #13078

Change-Id: Ie891d905d9c77f6ecadf7dcd5b44b477f4e079e0
Reviewed-on: https://go-review.googlesource.com/16360
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/vcs.go

@@ -122,8 +122,8 @@ var vcsGit = &vcsCmd{
 	name: "Git",
 	cmd:  "git",
 
-	createCmd:   []string{"clone {repo} {dir}", "--git-dir={dir}/.git submodule update --init --recursive"},
-	downloadCmd: []string{"pull --ff-only", "submodule update --init --recursive"},
+	createCmd:   []string{"clone --depth=1 {repo} {dir}", "--git-dir={dir}/.git submodule update --init --recursive"},
+	downloadCmd: []string{"pull --depth=1 --ff-only", "submodule update --init --recursive"},
 
 	tagCmd: []tagCmd{
 		// tags/xxx matches a git tag named xxx