Bug report - Can create worktrees from bare repo / such worktrees can fool is_bare_repository()

Hi folks! See the following bug report. Let me know if anything is
unclear -- in all honesty, I neglectfully `git worktree remove
--force`'d the first one I wrote...

Thank you for filling out a Git bug report!
Please answer the following questions to help us understand your issue.

What did you do before the bug happened? (Steps to reproduce your issue)

    ~$ git clone --bare https://github.com/git/git.git
    ---clip---

    ~/gitbare$ git config --list --show-origin
    file:config     core.repositoryformatversion=1
    file:config     core.filemode=false
    file:config     core.bare=true
    file:config     core.ignorecase=true
    file:config     remote.origin.url=https://github.com/git/git.git

    ~/gitbare$ git worktree add --no-checkout ../next
    Preparing worktree (checking out 'next')

    ~/gitbare$ git config --list --show-origin
    file:config     core.repositoryformatversion=1
    file:config     core.filemode=false
    file:config     core.bare=true
    file:config     core.ignorecase=true
    file:config     remote.origin.url=https://github.com/git/git.git

    ~/gitbare$ cd ../next/

    ~/next$ git config --list --show-origin
    file:../gitbare/config    core.repositoryformatversion=1
    file:../gitbare/config    core.filemode=false
    file:../gitbare/config    core.bare=true
    file:../gitbare/config    core.ignorecase=true
    file:../gitbare/config    remote.origin.url=https://github.com/git/git.git

    ~/next$ git rev-parse --is-bare-repository
    false

    ~/next$ git config extensions.worktreeconfig true
    ~/next$ git rev-parse --is-bare-repository
    true

    ~/next$ git config --unset extensions.worktreeconfig
    ~/next$ git rev-parse --is-bare-repository
    false

I actually found this situation (and narrowed it to the above) by
trying to perform a sparse-checkout in the worktree.  It appears
sparse-checkout by default uses a worktree-specific config (which
does make sense).

What did you expect to happen? (Expected behavior)

    I expected one of the following to happen:

    1. I should have been blocked from creating a worktree from a bare
    repository.

    2. is_bare_repository() shouldn't be fooled by this situation,
    assuming it's valid.

    All things being equal, I would more expect to have been blocked
    from creating a worktree from a bare repository.  Personally, this
    bare repo + worktree setup doesn't not align with my experience so
    far with how bare repos are normally used (i.e., as a convenience
    for centralized remotes that will never be doing a checkout).

What happened instead? (Actual behavior)

    is_bare_repository() is fooled and I'm prevented from performing
    any operation that requires a worktree (like performing a sparse
    checkout).

What's different between what you expected and what actually happened?

    is_bare_repository() is fooled into thinking the worktree is not a
    worktree / I'm able to create a worktree from a bare repo.

Anything else you want to add:

Please review the rest of the bug report below.
You can delete any lines you don't wish to share.


[System Info]
git version:
git version 2.34.1
cpu: x86_64
no commit associated with this build
sizeof-long: 8
sizeof-size_t: 8
shell-path: /bin/sh
uname: Linux 5.4.72-microsoft-standard-WSL2 #1 SMP Wed Oct 28 23:40:43
UTC 2020 x86_64
compiler info: gnuc: 9.3
libc info: glibc: 2.31
$SHELL (typically, interactive shell): /bin/bash


[Enabled Hooks]
not run from a git repository - no hooks to show

Best,
-Sean