Improve check copies (#19829)

* print first diff line intead of first code part line

* fix style

utils/check_copies.py

@@ -228,7 +228,12 @@ def is_copy_consistent(filename, overwrite=False):
 
         # Test for a diff and act accordingly.
         if observed_code != theoretical_code:
-            diffs.append([object_name, start_index])
+            diff_index = start_index + 1
+            for observed_line, theoretical_line in zip(observed_code.split("\n"), theoretical_code.split("\n")):
+                if observed_line != theoretical_line:
+                    break
+                diff_index += 1
+            diffs.append([object_name, diff_index])
             if overwrite:
                 lines = lines[:start_index] + [theoretical_code] + lines[line_index:]
                 line_index = start_index + 1