internal/testenv: allow Apple diff too

On recent versions of MacOS, tests in difftest and rename fail
because HasTool doesn't like the default Apple diff. The comment
(from 6 years ago) says that diff must support the -u flag,
for unified output formatting) and report missing newlines at the
end of files. The current Apple diff does this.

(I do not know why this does not affect the builders.)

Change-Id: I224a224eec1473e2a4bf3c947da77a8b3a623c9a
Reviewed-on: https://go-review.googlesource.com/c/tools/+/719920
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Madeline Kalil <mkalil@google.com>

Author: pjweinbgo

internal/testenv/testenv.go

@@ -131,10 +131,14 @@ func HasTool(tool string) error {
 		}
 
 	case "diff":
-		// Check that diff is the GNU version, needed for the -u argument and
+		// Check that diff is the GNU or Apple version, needed for the -u argument and
 		// to report missing newlines at the end of files.
 		out, err := exec.Command(tool, "-version").Output()
 		if err != nil {
+			out, _ = exec.Command(tool, "--version").Output()
+			if bytes.Contains(out, []byte("Apple diff")) {
+				return nil
+			}
 			return err
 		}
 		if !bytes.Contains(out, []byte("GNU diffutils")) {