cmd/gerritbot: skip a test when git isn't available

Part of getting x/build passing on the builders.

Change-Id: I549b3b8b8d290a8955adf19630aa10f2772da719
Reviewed-on: https://go-review.googlesource.com/c/157440
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/cmd/gerritbot/gerritbot_test.go b/cmd/gerritbot/gerritbot_test.go
index 0f81a48..cc8f3cb 100644
--- a/cmd/gerritbot/gerritbot_test.go
+++ b/cmd/gerritbot/gerritbot_test.go
@@ -5,6 +5,7 @@
 package main
 
 import (
+	"os/exec"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -43,6 +44,9 @@
 }
 
 func TestCommitMessage(t *testing.T) {
+	if _, err := exec.LookPath("git"); err != nil {
+		t.Skipf("skipping; 'git' not in PATH")
+	}
 	testCases := []struct {
 		desc     string
 		pr       *github.PullRequest