bug#58169: 29.0.50; comint-fl-mode fontifies the output

[Augusto Stoffel]

On emacs -Q, calling M-x shell and typing, say

  echo '#123'

prints some text with font-lock-comment-face.  Similarly

  echo '`123`'

prints text with the `sh-quoted-exec` face.  I wouldn't expect these
faces to be there, since the output is just output, not a comment or a
quoted exec.  If I turn off comint-fl-mode, then the output is not
fontified.