rustdoc: Print a warning if the diff when comparing to old nightlies is empty

This avoids confusing situations where it's unclear whether there's a
bug in the diff tool or not:

```
26: @Has check failed
        `XPATH PATTERN` did not match
        // @Has - '//code/a[@href="{{channel}}/std/primitive.i32.html"]' 'i32'

Encountered 6 errors

------------------------------------------

info: generating a diff against nightly rustdoc

failures:
    [rustdoc] rustdoc/primitive-reexport.rs
```

Author: jyn514

src/tools/compiletest/src/runtest.rs

@@ -2487,6 +2487,7 @@ impl<'test> TestCx<'test> {
 
         {
             let mut diff_output = File::create(&diff_filename).unwrap();
+            let mut wrote_data = false;
             for entry in walkdir::WalkDir::new(out_dir) {
                 let entry = entry.expect("failed to read file");
                 let extension = entry.path().extension().and_then(|p| p.to_str());
@@ -2499,17 +2500,28 @@ impl<'test> TestCx<'test> {
                         if let Ok(s) = std::fs::read(&expected_path) { s } else { continue };
                     let actual_path = entry.path();
                     let actual = std::fs::read(&actual_path).unwrap();
-                    diff_output
-                        .write_all(&unified_diff::diff(
-                            &expected,
-                            &expected_path.to_string_lossy(),
-                            &actual,
-                            &actual_path.to_string_lossy(),
-                            3,
-                        ))
-                        .unwrap();
+                    let diff = unified_diff::diff(
+                        &expected,
+                        &expected_path.to_string_lossy(),
+                        &actual,
+                        &actual_path.to_string_lossy(),
+                        3,
+                    );
+                    wrote_data |= !diff.is_empty();
+                    diff_output.write_all(&diff).unwrap();
                 }
             }
+
+            if !wrote_data {
+                println!("note: diff is identical to nightly rustdoc");
+                assert!(diff_output.metadata().unwrap().len() == 0);
+                return;
+            } else if self.config.verbose {
+                eprintln!("printing diff:");
+                let mut buf = Vec::new();
+                diff_output.read_to_end(&mut buf).unwrap();
+                std::io::stderr().lock().write_all(&mut buf).unwrap();
+            }
         }
 
         match self.config.color {