They provide a link to the section where they elaborate on “commit first vs test first”, here is the relevant text
Instead of jumping straight to the commit step, Fossil applies the proposed merge to the local working directory only, requiring a separate check-in step before the change is committed to the repository. This gives you a chance to test the change first, either manually or by running your software’s automatic tests. (Ideally, both!) Thus, Fossil doesn’t need rebase, squashing, reset --hard, or other Git commit mutating mechanisms
They provide a link to the section where they elaborate on “commit first vs test first”, here is the relevant text
Git has pre-commit hooks?
Git has a bunch of hooks. https://git-scm.com/book/en/v2/Customizing-Git-Git-Hooks
And
git merge --no-committo do whatever you want on the proposed merge before actually creating the commit. Test or whatever else.