Fix spurious rebuilds of generated go code

gen-go-code.py

@@ -4,6 +4,7 @@
 import io
 import json
 import os
+import subprocess
 import sys
 from contextlib import contextmanager, suppress
 from functools import lru_cache
@@ -295,7 +296,7 @@ var DocTitleMap = map[string]string{serialize_go_dict(ref_map['doc'])}
 # Boilerplate {{{
 
 @contextmanager
-def replace_if_needed(path: str) -> Iterator[io.StringIO]:
+def replace_if_needed(path: str, show_diff: bool = False) -> Iterator[io.StringIO]:
     buf = io.StringIO()
     yield buf
     orig = ''
@@ -305,6 +306,11 @@ def replace_if_needed(path: str) -> Iterator[io.StringIO]:
     new = f'// Code generated by {os.path.basename(__file__)}; DO NOT EDIT.\n\n' + new
     if orig != new:
         changed.append(path)
+        if show_diff:
+            with open(path + '.new', 'w') as f:
+                f.write(new)
+                subprocess.run(['diff', '-Naurp', path, f.name], stdout=open('/dev/tty', 'w'))
+                os.remove(f.name)
         with open(path, 'w') as f:
             f.write(new)