Always was puzzled by this "use present/imperative" prescription, to be fair. Shouldn't the messages about the past work be preterite/simple past? "Added support for OpenFizz 3.1", "Fixed FOO-2123 with foos that don't foo", "Changed default shell from /bin/fish to /bin/tcsh", etc. Heck, even the titular xkcd has initial commit messages in the Past Simple.
The way to think about it is you're not describing the work you've done, you're describing what the commit does.
It makes more sense that way if you think about the way in which Git is used for Linux development. They review individual commits so the commit message describes the change that will occur if that commit is pulled. Also commits can have multiple revisions as the change is improved, so past tense isn't correct because it can still change.
The "built-in" commit messages (e.g. "Revert ...") are imperative, so if nothing else it's consistent.
The way I read them is as statements of intent, i.e. "[this commit will] Merge/Revert/Add/Fix ...".