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

[Glenn Morris]

Kaushal Modi wrote:

> I personally have dealt with the issue where I cannot add untabify directly
> to write-file-functions hook, because untabify does not return nil.
>
> I need some sort of custom wrapper that runs untabify and then returns nil.

I'm pretty sure this has come up before.
IMO it's a feature. :)
Simply adding untabify to write-file-functions without thinking about it
(which not consulting the return value indicates) is a recipe for trouble.
Eg hope you like silently breaking every makefile you ever edit,
and irritating any other users of a shared VCS with whitespace changes.