tex-font-lock-suscript may mark buffer as modified

[Markus Rost]

Turn on global-font-lock-mode.  Create the following file a.tex:

===File ~/a.tex=============================================
$E=mc^2$
============================================================

Visit that file.  Call any of

M-x tex-mode
M-x latex-mode
M-x plain-tex-mode

The buffer becomes marked as modified (indicated by --:** in the mode
line) although you did nothing to edit the file.  I think this is a
bug.

Note that perhaps an auto save file is created and you will be asked
whether to save the buffer if you exit Emacs; this is sort of
annoying, since you did not really edit the file.

If you remove the superscript from the TeX file, as in

===File ~/b.tex=============================================
$E=mc2$
============================================================

this does not happen.  I guess the problem originates from
tex-font-lock-suscript which is used in tex-font-lock-keywords-3.  I
have no idea whether this is a problem of tex-mode or of font-lock.