Git provides the HEAD pointer to reference the currently checked out branch. Locally, this would be whichever branch you're on, but for a remote Git repo, this can be the default branch to check out the repo for.

A number of Git hosting services are exposing this recently as origin/HEAD, and I've found it to be such a useful tool, especially when I'm working across repos that have a different default branch configured.

I'm finding it incredibly helpful to replace my muscle memory of git rebase -i origin/develop or git rebase -i origin/main with git rebase -i origin/HEAD, which saves me having to remember what branch is currently used!

Sometimes you clone a fresh repo, before a default branch is configured (i.e. if it's empty), and that leads to origin/HEAD never being updated, as it doesn't pull that information as part of a git fetch.

When the remote repo exposes origin/HEAD

You may be fortunate with the repo host producing the origin/HEAD ref, in which case, you can run the following to sync your local origin/HEAD with them:

git remote set-head origin -a

When the remote repo does not expose origin/HEAD

If it's not set, we need to do it ourselves. Assuming that the Git repo uses the trunkasaurus branch:

git remote set-head origin trunkasaurus

This also works when you don't agree with what's being told to you as default, and you want a different value locally!