bug#20290: 25.0.50; Use unified diff format by default, and recommend using it in reports and patches

[Dmitry Gutov <dgutov@yandex.ru>]

On 04/13/2015 02:30 AM, Paul Eggert wrote:
> As I recall, RMS preferred diff -c to diff -u, and that is partly why
> trouble.texi suggests -c over -u and why diff-switches defaults to -c.

Yup, Stephen reminded of that in a recent emacs-devel thread, so I 
thought this would be a good time to try to change it.

> These days -u format is far more popular than -c format in practice, and
> I expect that most active Emacs developers prefer -u, so it's
> appropriate for Emacs to default to -u and for trouble.texi to suggest it.

This is done now. Thanks everyone.

> I suppose that in theory there is the issue of Emacs running on an older
> POSIXish platform where 'diff' does not support -u.  I don't have access
> to any such platform, though.  I can confirm that Solaris 10 diff (2005)
> supports '-u'.  So perhaps we don't need to worry about this.

I think the usual response is that users on outdated systems don't 
necessarily have to be able to use the latest-and-greatest Emacs.

But in this case the incompatibility would be very minor: if someone 
actually encounters this problem, they'll only need to customize the 
relevant variable.