GitHub Actions: use GitHub Checks for line comments support

<!-- markdown --> Users: Comments added on a per line basis were only ever shipped with support on GitHub, only on PRs, and was rarely used due to being very noisy. That functionality has been removed and replaced with [GitHub Checks Annotations]( https://github.blog/2018-12-14-introducing-check-runs-and-annotations/ ) when run under [GitHub Actions]( https://github.com/features/actions ) with support for both branches and PRs. Any 3rd party plug-ins that support line-level comments will need to be updated to support the new API and file naming conventions in order to be compatible. Developers: With GitHub Checks Annotations being the preferred way on GitHub for CI systems to make individual notes, the API requirements and surrounding support structure for line comments have also changed: * File names MUST be relative to `BASEDIR`. * The unidiff line calculations (`compute_unidiff`) has been removed. This should greatly increase performance for large patches. * Relatedly, `GITUNIDIFFLINES` has been removed from the global variables. * `(bugsystem)_linecoments` and related functions should now expect and pass a column parameter. Log files should be in typical file:line[:column]:text format, where the missing column or 0 or "" will be used for no column/whole file. * `BUGLINECOMMENTS` is no longer wiped when doing a diff vs. patch. * `(bugsystem)_linecomments` will no longer receive comments grouped by file names. Implementers will need to do their own grouping, if required. * A new `(bugsystem)_linecomments_end` will be called when all line comments have been processed if there is any final work that needs to be done.