add helpful message and test

pre_commit/commands/run.py

@@ -215,7 +215,14 @@ def _run_hooks(config, hooks, args, environ):
         if retval and config['fail_fast']:
             break
     if retval and args.show_diff_on_failure and git.has_diff():
-        output.write_line('All changes made by hooks:')
+        if args.all_files:
+            output.write_line(
+                'Pre-commit hook(s) made changes. '
+                'If you are seeing this message on CI,'
+                ' reproduce locally with: pre-commit run --all-files',
+            )
+        else:
+            output.write_line('All changes made by hooks:')
         subprocess.call(('git', '--no-pager', 'diff', '--no-ext-diff'))
     return retval
 

tests/commands/run_test.py

@@ -178,16 +178,41 @@ def test_global_exclude(cap_out, store, tempdir_factory):
         assert printed.endswith(expected)
 
 
-def test_show_diff_on_failure(capfd, cap_out, store, tempdir_factory):
+@pytest.mark.parametrize(
+    ('args', 'expected_out'),
+    [
+        (
+            {
+                'show_diff_on_failure': True,
+            },
+            b'All changes made by hooks:',
+        ),
+        (
+            {
+                'show_diff_on_failure': True,
+                'all_files': True,
+            },
+            b'reproduce locally with: pre-commit run --all-files',
+        ),
+    ],
+)
+def test_show_diff_on_failure(
+        args,
+        expected_out,
+        capfd,
+        cap_out,
+        store,
+        tempdir_factory,
+):
     git_path = make_consuming_repo(
         tempdir_factory, 'modified_file_returns_zero_repo',
     )
     with cwd(git_path):
         stage_a_file('bar.py')
         _test_run(
-            cap_out, store, git_path, {'show_diff_on_failure': True},
+            cap_out, store, git_path, args,
             # we're only testing the output after running
-            (), 1, True,
+            expected_out, 1, True,
         )
     out, _ = capfd.readouterr()
     assert 'diff --git' in out