Configure/use upstream/pushRemote when pushing and pulling

[altsem]

@rynoV I've been hacking around and testing things out. I opened up this new PR, but it continues on your work. I'd appreciate any input on the code! :)

To summarize:

• Change bindings to u and p: p was previously the push.default (which often would be upstream). Now u is upstream, and p is pushRemote
• Fix when upstream was a local branch
• Make push-commands more explicit with refspecs (I copied Magit)
• Display upstream & pushRemote dynamically in the menus: to origin/main instead of to upstream