Add maktaba logging for whenever codefmt modifies buffer

It would be useful to log an info message whenever codefmt triggers and modifies the buffer, especially for cases where it triggers automatically on save.

It should be possible to verify whether the buffer was actually changed by checking `b:changedtick` before and after running the formatter so we can avoid logging a message if the formatter had no effect.

Start by locating the codefmt formatting entry point and the automatic-on-save path. Check b:changedtick before and after formatting, then verify that an info message appears only when the buffer changed; test both changed and unchanged cases.

Written by the indexing model from the issue text.