Allow flexible trunk branch for `git-up`
Allow flexible trunk branch for `git-up`

Our team is renaming the base branch of our application from `master` to
`trunk`. I had held off on doing this in the past because `git up -i`
has become such an huge muscle memory thing and I didn't want a script
to only work on some of my frequent repositories.

This [trick from Stack Overflow](https://stackoverflow.com/a/49384283/1190970)
shows how to grab the name of the primary branch for a remote branch without an
expensive git operation. It'll allow a rebase against the tip of the primary
remote branch regardless of its name.

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