Confusing error message due to Git trusting is_bare_repository()

I had a co-worker git this well-known error message:

    error: refusing to update checked out branch: refs/heads/master
    error: By default, updating the current branch in a non-bare repository
    error: is denied, because it will make the index and work tree inconsistent
    error: with what you pushed, and will require 'git reset --hard' to match
    error: the work tree to HEAD.

It *was* a bare repo, but the config file had bare=false because
someone had simply copied a .git tree from a non-bare repo to make it.

The issue is that we just use this:

    int is_bare_repository(void)
    {
        /* if core.bare is not 'false', let's see if there is a work tree */
        return is_bare_repository_cfg && !get_git_work_tree();
    }

For the purposes of the error message it would be helpful if we also
detected whether something didn't have a working tree, but was set to
bare=false, and tell the user to updatet he bare=false to bare=true
for his almost-bare repository.