bug#53729: 29.0.50; Tamil text not shaped in modeline

[Eli Zaretskii <eliz@gnu.org>]

> From: Jai Vetrivelan <jaivetrivelan@gmail.com>
> Cc: Visuwesh <visuweshm@gmail.com>,  53729@debbugs.gnu.org
> Date: Sat, 19 Feb 2022 09:50:10 +0530
> 
> I tried reproducing the above issue and can confirm it.
> 
> The text is weirdly shaped when opening a binary file with file
> extension. There are no messages in stdout/stderr, however.

Thanks.  I think I see the problem.  If I'm right, file-name
extensions are not related to this.

Please try this much simpler reproducer:

  emacs -Q
  C-x b கட்டற்ற மென்பொருள் RET

At this point you should see a buffer whose name is "கட்டற்ற மென்பொருள்"
and whose mode line displays the buffer name correctly.

  M-: (set-buffer-multibyte nil) RET

Now the buffer's name should display incorrectly on the mode line.

IOW, the problem happens when the buffer is unibyte, probably because
we disable text shaping (a.k.a. "character compositions") in that
case.