`git_branch_upstream` could not get the upstream branch in `config.worktree`

[Whirlwind]

Reproduction steps

$ git clone --filter=blob:none git@xxx:xx/xx.git
$ cd xx
$ git worktree add ../worktree
$ cd ../worktree
$ git config extensions.worktreeConfig true
$ git checkout new-branch
$ git push -u

Then, use the git_branch_upstream will could not get the origin/new-branch, it said GIT_ENOTFOUND.

Expected behavior

git_branch_upstream can get the upstream branch in the config.worktree.

Actual behavior

throw error.

Version of libgit2 (release number or SHA1)

1.6.4

Operating system(s) tested

macOS 14.1

[csware]

cf. https://github.com/libgit2/libgit2/issues/6044

[ethomson]

Should be fixed via #6756