Branches at the root commit lead to confusing errors; jj abandon can create such a branch

[ilyagr]

To do it, simply add a branch to a child of the root commit and then jj abandon the branch.

Branches at the root commit cause confusing errors on git export and git push.

Since #1479, jj branch is not allowed to create such branches. If we decide to resolve this by supporting such branches better, that restriction should be lifted.

[martinvonz]

Can we make the error less confusing? I don't see anything inherently bad about having a branch on the root commit.

[ilyagr]

That would work too. It'd be a bit of work, since we'd have to figure out how to export such branches (we could ignore them or we could try to put the git repo in the state git checkout --orphan puts it in) and figure out when the user needs to be informed that their branch is not representable in git (for example, it cannot be pushed).

Update: I rewrote the bug description to leave more possibility for such a resolution.

[martinvonz]

We already skip conflicted branches on exported. Maybe we can treat these branches similarly (but ideally producing a different warning message for the user).