Dmitry Gutov wrote:

> - Shouldn't that change be in vc-coding-system-for-diff?
> - It seems to try to fix a separate issue (whether all files use the same coding
> system).

Yes. For emacs-25 that's probably too much, as you suggest. So we can fix the 
problem in vc-coding-system-for-diff. Revised (more-conservative) patch attached.

> - Like Eli pointed out, (coding-system-get coding-system-for-read
> :ascii-compatible-p) should work about as well. Why doesn't it?

It doesn't work for EBCDIC.

> As an aside, how did you manage to create a patch that's using tabs for
> indentation, with indent-tabs-mode bound to nil in .dir-locals.el? That's
> troubling.

I override that setting, as I find it annoying in too many cases. It's just a 
minor annoyance, but there it is.