cmd/go: fix git submodule fetch

Thanks to @toxeus on GitHub for the test case.

Fixes #12612.

Change-Id: I0c32fbe5044f3552053460a5347c062568093dff
Reviewed-on: https://go-review.googlesource.com/17974Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/vcs.go

@@ -122,7 +122,7 @@ var vcsGit = &vcsCmd{
 	name: "Git",
 	cmd:  "git",
 
-	createCmd:   []string{"clone {repo} {dir}", "--git-dir={dir}/.git submodule update --init --recursive"},
+	createCmd:   []string{"clone {repo} {dir}", "-C {dir} submodule update --init --recursive"},
 	downloadCmd: []string{"pull --ff-only", "submodule update --init --recursive"},
 
 	tagCmd: []tagCmd{

src/cmd/go/vendor_test.go

@@ -187,6 +187,18 @@ func TestVendorGetUpdate(t *testing.T) {
 	tg.run("get", "-u", "github.com/rsc/go-get-issue-11864")
 }
 
+func TestGetSubmodules(t *testing.T) {
+	testenv.MustHaveExternalNetwork(t)
+
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.makeTempdir()
+	tg.setenv("GOPATH", tg.path("."))
+	tg.setenv("GO15VENDOREXPERIMENT", "1")
+	tg.run("get", "-d", "github.com/rsc/go-get-issue-12612")
+	tg.run("get", "-u", "-d", "github.com/rsc/go-get-issue-12612")
+}
+
 func TestVendorCache(t *testing.T) {
 	tg := testgo(t)
 	defer tg.cleanup()