Set branch upstream

Many tools expect to be able to find the associated upstream for a given branch; `gh stack` currently manages that internally.

When one does `gh stack init some-branch`, then at such time as `gh stack` actually pushes the branch to `origin`, it should do the equivalent of `git push --set-upstream origin some-branch` in order to create the association.

This lets the user push future changes with just `git push` (and check for updates with `git pull`), as well as letting `git status` show relative differences.

Trace the `gh stack init` and push flow in the Go CLI, starting where a branch is pushed to `origin`. Confirm the resulting behavior with a stacked branch: the first push should establish its upstream, and subsequent `git push`, `git pull`, and `git status` commands should use or report that association.

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