cmd/go: skip gitrepo tests on Plan 9

CL 118095 added gitrepo tests. These tests are failing on Plan 9
since they expect a full-featured git command, while the git tool
has been emulated as a simple rc script on Plan 9.

Fixes #25938.

Change-Id: I262a89d0ce83168c550d9af3e832ed3a1e3c43f6
Reviewed-on: https://go-review.googlesource.com/122455
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/internal/modfetch/gitrepo/fetch_test.go

@@ -16,6 +16,7 @@ import (
 	"os/exec"
 	"path/filepath"
 	"reflect"
+	"runtime"
 	"strings"
 	"testing"
 	"time"
@@ -42,6 +43,12 @@ func testMain(m *testing.M) int {
 		return 0
 	}
 
+	if runtime.GOOS == "plan9" {
+		fmt.Fprintln(os.Stderr, "skipping on plan9")
+		fmt.Println("PASS")
+		return 0
+	}
+
 	dir, err := ioutil.TempDir("", "gitrepo-test-")
 	if err != nil {
 		log.Fatal(err)