cloudfns/sendwikidiff: fix test when git not available (like on windows longtest)

Change-Id: I4277f8b9d3872cc14149788df5a2bb02a7eeeb2d
Reviewed-on: https://go-review.googlesource.com/c/build/+/211099
Reviewed-by: Bryan C. Mills <bcmills@google.com>
diff --git a/cloudfns/sendwikidiff/sendwikidiff_test.go b/cloudfns/sendwikidiff/sendwikidiff_test.go
index 239f6fa..9e69012 100644
--- a/cloudfns/sendwikidiff/sendwikidiff_test.go
+++ b/cloudfns/sendwikidiff/sendwikidiff_test.go
@@ -7,6 +7,7 @@
 import (
 	"context"
 	"os"
+	"os/exec"
 	"testing"
 )
 
@@ -14,6 +15,9 @@
 	if testing.Short() {
 		return
 	}
+	if _, err := exec.LookPath("git"); err != nil {
+		t.Skipf("git binary not available")
+	}
 	oldSendgridKey := sendgridAPIKey
 	sendgridAPIKey = "super secret key"
 	oldSendEmail := sendEmail