spinoff can look at push branches too

[glasserc]

I often wish I had something like magit-branch-spinoff but that respected pushRemote instead of merely checking the "upstream" of the branch. I tried to surface this need in #4822 but today I looked more closely at the code and I thought I could see a way to implement it easily, so I thought I would upgrade my discussion into a PR.

I tried to test this by:

• Using C-x C-e on my changed version
• Going to a repo with a feature branch that I had pushed
• Adding an empty commit to that branch with the message "Test"
• Using b s to create a new branch called bloop
• Viewing the log to verify that bloop contained the new commit and was based off the pushed state of the feature branch
• Switching to the feature branch and verifying that it no longer had any unpushed commits

Thanks for your efforts on magit!

[tarsius]

Funny enough I was thinking about possible improvements for this family of commands, just hours before you opened this. But I decided delaying any work until after the next release, to not add yet another "just one more thing" that would contribute to indefinitely delaying the release.