I am sorry….how scary. I would probably carry a walking stick that I could try to fend off the other dog. It may very well be that the other dog was trying to initiate play...the fact that you could grab the dog on the neck and it didn't attack you, indicates it wasn't actually being aggressive. But still very scary, none the less. The down side to trying to intervene with a stick between two dogs, is the possiblity that you could make an approaching dog angry, and cause a fight.....
You might send a note to the dog's owners that explain that your dog was very frightened when their dog bolted out of the house; if she is nervous about walks now, tell them that. And politely mention that if it happens again, you will contact the authorities. To encourage Bella to be more confident, bring some treats, and feed her as you get to the area where she is nervous.
Pits and other bully dogs can be great, great dogs but their owners have to be super diligent about their care....they can't allow them to dart out the door, or jump the fence...it is just too frightening to give them some of the liberties that smaller, less dangerous dog are allowed.