Is your feature request related to a problem? Please describe.

I quite often switch between working on my laptop and my desktop. When I switch, I tend to run jj git fetch first. If I'm responding to a PR review, I want to switch to the branch of that PR, which requires me to track it. I use autogenerated branch names, so I usually copy the branch name from GitHub (using the little clipboard button). I then want to type jj branch track <PASTE>, but annoyingly I often forget to add @origin and get an error and have to retry

Describe the solution you'd like

jj branch track foo would default to origin, or maybe any remote if it's unambiguous.