Disclaimer: just about anything can go wrong with the answer below.
Since no one has to encroach upon your personal space just to ask the time, I would make sure I kept my cart between him and me. I would tell him the time, not by looking at my watch, because I'm not breaking eye contact for any reason, but by providing a rough estimate. If he continues to close with hands exposed and w/out a weapon in sight, I will defend myself, not with a gun; but with the training I received over five years in a martial arts studio. I'm not going to shoot an unarmed assailant, period. If you do, there's a better than average change that you will be spending time behind bars.
If there were two strangers closing the distance, I'd jump on top of the car. Once I have firm footing, I'd have my hand near my gun. Assuming no one was around, and the two men crossed the line from innocent stranger to obvious aggressor, I'd still scream my lungs out, in hopes of either scaring them off with my singing voice or alerting someone out of sight.
If the attackers were still determined to escalate the situation, but had not presented a weapon of any kind, I'd quickly determine if I could fight them off. Considering the fact that I have the higher ground, perched atop my car, a kick or two to the face is a good tactic.
At the first sight of a weapon from either assailant, the gun leaves its holster and I eliminate the primary threat before turning on the secondary one. If the remaining aggressor is still unarmed, I do not shoot but I do keep yelling in order to raise the alarm. Chances are very good that the remaining attacker leaves after seeing his partner spouting blood; but if not, I will be prepared to continue the fight, will my hands and feet, or with a gun, depending on the circumstances.
And they lived happily ever after