Not the answer you’re looking for, but since I never found a fix to this, and since the site I was working on only had simple products, I edited out the Edit button on the cart page. There’s not much need for it, if at all, b/c simple products have nothing to edit. The only option is to edit the quantity, which they can do right from the cart page, and less options is actually preferable here. Configurable products is another story.....
I’m assuming it’s an unfinished new feature that made it into the commit
Pretty annoying having to explain to a client - after they have seemingly found a ‘broken feature’ (that they wouldn’t necessarily have expected to be there originally) you have to then explain to them that they won’t get this feature anymore unless they want to pay for an extension…
Oh well, one should never complain about open source software
The edit feature on the cart page requires an addition to your layout’s checkout.xml. You can read about that here and attempt their suggested addition to your checkout.xml:
Magento Cart Edit Upgrade Fix
Or, (my recommendation) you can delete your theme’s layout checkout.xml so it works off the newly upgraded base’s checkout.xml.
If you had modifications in your checkout.xml that you still need, hopefully not too many, you may be better off moving those local.xml.