How to tell that buffer is not modifed?

[Jean Louis <bugs@gnu.support>]

I would like to tell programmatically that buffer is not modified even
though it is modified.

I am inserting the database entry into the buffer, but because
`set-auto-mode' works only on buffers with associated file names, I am
assocating a file name with revision number. This association makes
buffer look modified and killing buffer asks for saving. I would like
to tell that buffer is not modified, as the information arrived from
database and it is anyway in a read only mode.

    (switch-to-buffer (get-buffer-create buffer))
    (setq-local buffer-file-name filename-revised)
    (read-only-mode 0)
    (insert body)
    (goto-char 1)
    (set-auto-mode)
    (read-only-mode 1)))

Jean