`batch-mark-files-as-viewed` only works in one direction #3549

1. Visit https://github.com/sindresorhus/refined-github/pull/3540/files
2. Click "Viewed" on the 1st file
3. Shift-click "Viewed" on the 3rd file

This works, 3 files are selected.

4. Remove all "Viewed" marks
5. Click "Viewed" on the 3rd file
6. Shift-click "Viewed" on the 1st file

This doesn't work. Only 1 or 2 files are selected, not 3 as expected.

Whoever wrote this feature was a real dummy. Let me see who did...

``` $ git blame source/features/batch-mark-files-as-viewed.tsx Fregante (4): Add batch-mark-files-as-viewed feature (#3007) Extract page-detect.ts into github-url-detection (#3062) Lint (#3072) Meta: Reorganize utility functions (#3110) .--
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