bug#21492: 25.0.50: Make untabify work nicely with write-file-functions

[Stefan Monnier]

> I also have now started using before-save-hook instead of
> write-file-functions where I don't care about the return values of the
> functions added to the hook.

BTW, the use of before-save-hook is not just so you don't have to care
about the return value.  It's also because some functions on
write-file-functions may not be run at all (if some earlier function
returns non-nil because it took care of performing the write).

So even adding a "nil" at the of your function doesn't make it right to
add it to write-file-functions.


        Stefan