CMS5: Replace thirdparty difflib

[GuySartorelli]

As part of #10488 it has been determined that we should replace the thirdparty difflib with a maintained library which should be added via composer.

A good candidate is sebastianbergmann/diff, and I have created a proof of concept PR which does some of the work to implement this library - but take a look at #10488 (comment) for other possible alternatives.

difflib is only directly referenced in SilverStripe\View\Parsers\Diff which subclasses it - but that class is in turn referenced in a few places:

• Various tests reference it
• SilverStripe\VersionedAdmin\Forms\DiffField in silverstripe/versioned-admin uses it (compareHTML() only, none of the underlying methods from difflib, so should be fine)
• SilverStripe\Versioned\DiffDifferencer uses it (again using compareHTML() only, none of the underlying methods from difflib)

Acceptance Criteria

• The thirdparty difflib is removed from the codebase, and a suitable replacement is added via composer
• The existing functionality which uses difflib is changed to use the replacement library
• The result is clearly correct, even if slightly different to the output from difflib
  • e.g. it's okay if, when replacing content, the removal is shown before the addition where difflib shows the addition before the removal
• The changelog calls out that the SilverStripe\View\Parsers\Diff class no longer has all of the methods it was inheriting from difflib
• The methods in SilverStripe\View\Parsers\Diff which were inherited from difflib are deprecated in CMS 4 Changed\Removed methods defined directly in SilverStripe\View\Parsers\Diff are deprecated in CMS 4
  • Methods inherited from the thirdparty difflib are not called out as deprecated methods.

PRs

CMS 5

• API Replace thirdparty difflib with maintained lib #10641
• FIX Use the new HtmlDiff class silverstripe-versioned#392
• FIX Use the new HtmlDiff class silverstripe-versioned-admin#272

CMS 4 (deprecations)

• API Deprecate Diff in favour of CMS5's HtmlDiff #10677
• ENH Avoid deprecation warnings for deprecated Diff silverstripe-versioned#393
• ENH Avoid deprecation warnings for deprecated Diff silverstripe-versioned-admin#273

[GuySartorelli]

Note that the acceptance criteria for updating the changelog will be done as a part of the automation in https://github.com/silverstripeltd/product-issues/issues/627

[sabina-talipova]

Third party difflib library was replaced successfully. Issue is closed.