Remove unmaintained dependency go-difflib

[Crevil]

The package github.com/pmezard/go-difflib is as of this commit no longer maintained: https://github.com/pmezard/go-difflib/commit/1ef1646f464bff6853dfa4c32750886762dc023a

Maybe we could move the used functionality into this repo and stop depending on the difflib package all together?

The functions used are difflib.GetUnifiedDiffString() and difflib.SplitLines().

[kkaushy]

Is there any update on this.

[dolmen]

Who will maintain that imported code?

What are the issues in difflib code that need fixes?

So far, as a maintainer, I don't see any advantages compared to the status quo.

[dolmen]

Duplicate of #1187.