I have a pull request I would want to review locally in a git work tree:

$ git ls-remote origin 'pull/*/head'
d...    refs/pull/2/head

This pull request has a branch main. The documentation of git-worktree tells me that I can add a new worktree by using the following syntax:

add <path> [<commit-ish>]

Then again, a previous discussion on what is a <commit-ish> gives me at least the following alternatives for this ish-creature:

----------------------------------------------------------------------
|  Commit-ish/Tree-ish   |                Examples
----------------------------------------------------------------------
|  <refname>             | master, heads/master, refs/heads/master
|  <refname>@{upstream}  | master@{upstream}, @{u}

What is the correct format for <commit-ish> for this pull request?

Every format I have tried so far has failed, including – but not limited to – the following:

$ git worktree add ~/tmp/pr-2-review refs/pull/2/head
fatal: invalid reference: refs/pull/2/head
$ git worktree add ~/tmp/pr-2-review refs/pull/2/head@{upstream}
fatal: no such branch: 'refs/pull/2/head'
$ git worktree add ~/tmp/pr-2-review refs/pull/2/head/main@{upstream}
fatal: no such branch: 'refs/pull/2/head/main'
$ git worktree add ~/tmp/pr-2-review pull/2/head@{upstream}
fatal: no such branch: 'pull/2/head'
$ git worktree add ~/tmp/pr-2-review pull/2/head/main@{upstream}
fatal: no such branch: 'pull/2/head/main'