Diesel fuel is oily. I would try dumping cheap clay cat litter on the spot. Maybe even step on it and sort of "grind" it into the stain. Put down a pretty thick layer of the litter and leave it for a day or two. It should absorb some of the oil. Sweep it up and apply fresh litter again. It should continue to absorb more of the oil. Auto parts stores sell an oil absorbent called "Zip Zorb" which seems to be to be a fancy, specialized name for cat litter!
I would then scrub the spot with a stiff scrub brush and a strong solution of TSP (tri-sodium phosphate) and boiling hot water and then rinse with more boiling hot water. You can buy TSP at hardware stores. It's in powder or granular form. I've even absorbed oil stains with a layer of plain old cornstarch but I think cat litter would be cheaper and more practical for this application. Good Luck!
Yoy could try water soluble degreaser from a big box store. Scrub with a bristle brush and rinse.
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