FR: jj should play nice with gh cli - prioritize fetching from upstream when set over origin

[joshka]

Is your feature request related to a problem? Please describe.
In git, if an upstream repo is set, it's prioritized over origin when fetching.
When using the gh cli tool, letting the tool fork an origin repo for you renames origin to upstream and adds a new origin.

Describe the solution you'd like
Default fetch priority should be upstream first, origin second

Describe alternatives you've considered
If we don't do this, then users have to dig into documentation to work out how to set their local repos up for working with these tools well. This behavior is divergent from git's behavior without any obvious affordances for discovery. The failure mode here is that jj git fetch no longer does anything except report "nothing changed"

Additional context
Add any other context or screenshots about the feature request here.

[bnjmnt4n]

In git, if an upstream repo is set, it's prioritized over origin when fetching.

I don’t believe this is true.

From https://git-scm.com/docs/git-fetch:

When no remote is specified, by default the origin remote will be used, unless there’s an upstream branch configured for the current branch.

I believe it’s because the branch created when forking a repo using gh has its upstream branch configured specifically. This doesn’t apply to all Git projects.