[tools] presubmit.py: Print actual diff on presubmit failure

Change-Id: I174dd958c7854b0fa59228085bd23fe01cdf1fa0
Reviewed-on: https://chrome-internal-review.googlesource.com/c/v8/fuzzilli/+/9032276
Commit-Queue: Matthias Liedtke <mliedtke@google.com>
Reviewed-by: Michael Achenbach <machenbach@google.com>
Auto-Submit: Matthias Liedtke <mliedtke@google.com>

Author: Liedtke

Tools/presubmit.py

@@ -20,7 +20,10 @@ def check_git_clean():
         cwd=BASE_DIR,
         capture_output=True,
         check=True)
-    assert result.stdout.decode().strip() == "", f"Unexpected modified files: {result.stdout.decode()}"
+    output = result.stdout.decode().strip()
+    if output != "":
+        diff_result = subprocess.run(["git", "diff"], cwd=BASE_DIR, capture_output=True, check=True)
+        assert False, f"Unexpected modified files: {output}\n== Diff ==\n{diff_result.stdout.decode()}"
 
 def check_proto():
     """Check that program.proto is up-to-date."""