bug#5370: M-x hexl-mode M-x hexl-mode leaves unwanted ruler

[Juanma Barranquero <lekktu@gmail.com>]

  emacs -Q
  M-x hexl-mode <RET>
  y              ; to the question about discarding undo
  M-x hexl-mode <RET>
  C-c C-c

now *scratch* has a ruler, thanks to `header-line-format' containing

   (:eval (funcall ruler-mode-ruler-function))

while `ruler-mode' is nil (so M-x ruler-mode doesn't remove it).

The second call to hexl-mode bypasses most of the hexl-mode
initialization, but runs `hexl-mode-hook', which by default contains
`hexl-activate-ruler', so the ruler is initialized again. `ruler-mode'
already has code to save header-line-format, but it seems like it
isn't dealing too well with reentering.

So, I think the bug is in ruler-mode. But, what to do? Create a stack
of previous header-line-format's? At some point the thing turns
ridiculous. In the case above, however, it is a real bug (though
harmless) because the unwary user will have a hard time removing the
ruler.

Still, shouldn't hexl-activate-ruler be a noop when the buffer is
already in hexl-mode? It is certainly possible to do

  M-x hexl-mode  ; you have a hexl ruler
  M-x ruler-mode  ; to turn it off
  M-x hexl-mode  ; you get it back

but I can't really say whether the user would expect at that point to
have a hexl ruler or not in the buffer :-)

    Juanma