FR: Commands to prune branches from git repo when disabling colocation

[jennings]

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

When not using a colocated repository, but the backing repository contains branches (refs/heads/*), they show up as @git bookmarks and cannot be easily removed from jj.

This seems to happen most often after running jj git colocation disable, but it would also be a problem after accidentally running jj git export.

It's error-prone to remove these with git -C $(jj git root) branch -D ... because if somehow jj git import runs afterwards, jj will abandon the commits that are now unreachable (or will have conflicted bookmarks).

Describe the solution you'd like
I think 99% of the time, when disabling colocation, the user wants all heads removed because they plan to never use the Git repo. So, I think jj git colocation disable should automatically remove the branches.

If the user didn't want this, they could run jj git export (which they'll probably be doing often anyway).

Optionally, we could add a jj git export --prune that removes all refs/heads/* from the underlying git repo without abandoning any commits. This supports the scenario of running jj git export in a non-colocated repository.

Describe alternatives you've considered

• jj git colocation disable --prune-branches: I think pruning is wanted 99% of the time, so if anything this should be --no-prune-branches.
• jj bookmark delete <name>@git: This doesn't work for other "remote" bookmarks, so I'm not sure that we should make @git a special case.
• Anything like jj git export-prune or jj bookmark prune-git: These feel like very specialized commands, it may not be worth making them a "full" command.

Additional context

I'm opening an issue for discussion before actually implementing anything.

Related to:

• Stale @git bookmarks and revisions #8055
• https://discord.com/channels/968932220549103686/1532066356822933644/1532066356822933644

[josephlou5]

I think the best solution would probably be jj git export --prune that removes the refs as you said, and then jj git colocation disable should automatically run that as well (with an optional flag to disable such behavior). Essentially the best of both worlds!

Only one of these solutions seems not ideal to me:

• If only jj git export --prune existed, then perhaps running jj git colocation disable will print a hint saying "hey you might want to run jj git export --prune after this", which seems like extra work and thus not as good UX.
• If only jj git colocation disable existed, then if I run jj git export in a non-colocated repo, resulting in @git bookmarks, I will have to run jj git colocation enable && jj git colocation disable (since the latter command doesn't work if you're already not colocated).

Thus my conclusion to have something like jj git export --prune, which jj git colocation disable will automatically do on its own (so the user doesn't have to do it manually after).