[bug-diffutils] bug#24629: Make unified diff configurable default

[Norbert Kiesel]

Hi,

it's been ages that I actually wanted to produce a "classic" diff, and
I don't think I'm alone in that.  Yet, I still have to remember to add
`-u` every time I invoke `diff`.

I understand changing the default is controversial. But perhaps even
using "classic" as default unless POSIXLY_CORRECT is set.

But we should be able to support a GNU_DIFF_FORMAT environment
variable that sets the default format, no?

I could send a patch, but first want to understand if that would have
any chance of being applied.

</nk>

P.S.: I know I could write my own little wrapper script/alias/shell
function, but that all seems dirty in one way or another. --nk