git: how to restore a deleted worktree?

[Stephen Leake <stephen_leake@stephe-leake.org>]

I have the emacs-26 branch checked out in a worktree created by :

cd emacs/master
git worktree add -b emacs-26 ../emacs-26 origin/emacs-26

At some point, that worktree was in a confusing state, so I deleted it,
intending to do a fresh checkout. So I repeated the above command, but
it complained "branch emacs-26 already exists". So I dropped the '-b
emacs-26', and the command seemed to work.

However, 'git status' in the new worktree says "not on any branch", 
'git push' complains about fast-forward, and 'git pull' says not on a
branch.

How do I fix this?

-- 
-- Stephe