user/diffutils: new package

[wezm]

Description

Adds GNU diffutils. I need GNU diff for the test suite of the product I work on for my job. Unfortunately BSD diff and GNU diff have incompatible forms for grouping in regexs \(, \) for GNU, ( and ) for BSD, which we use. So I can't easily avoid the need for GNU diff without breaking things for other colleagues.

Checklist

Before this pull request is reviewed, certain conditions must be met.

The following must be true for all changes:

• I have read CONTRIBUTING.md

The following must be true for template/package changes:

• I have read Packaging.md
• I have built and tested my changes on my machine

The following must be true for new package submissions:

• I will take responsibility for my template and keep it up to date

[triallax]

not 100% sure about this but since we have gsed, ggrep, etc., maybe this should also be gdiffutils?

[wezm]

I think the 'g' prefix makes sense where there are other packages that might want to use the same name, but I'm not sure there's another 'diffutils'. The tools do get installed with the 'g' prefix though.