Merge branch 'dk/worktree-dup-checkout-with-bare-is-ok'

Merge branch 'dk/worktree-dup-checkout-with-bare-is-ok'

In a worktree connected to a repository elsewhere, created via "git
worktree", "git checkout" attempts to protect users from confusion
by refusing to check out a branch that is already checked out in
another worktree.  However, this also prevented checking out a
branch, which is designated as the primary branch of a bare
reopsitory, in a worktree that is connected to the bare
repository.  The check has been corrected to allow it.

* dk/worktree-dup-checkout-with-bare-is-ok:
  worktree: allow the main brach of a bare repository to be checked out