bug: setting GIT_DIR to $(git rev-parse --git-dir) changes behavior

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Please answer the following questions to help us understand your issue.

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

$ env | grep GIT
$ git --version
git version 2.30.1
$ git init t
Initialized empty Git repository in /tmp/t/.git/
$ mkdir t/foo
$ cd t/foo
$ git rev-parse --show-toplevel
/tmp/t
$ GIT_DIR=$(git rev-parse --git-dir) git rev-parse --show-toplevel
/tmp/t/foo
$ git rev-parse --git-dir
/tmp/t/.git

What did you expect to happen? (Expected behavior)

"git rev-parse --show-toplevel" should be the same in both invocation.

What happened instead? (Actual behavior)

"git rev-parse --show-toplevel" shows the current working directory if GIT_DIR is set.

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

Setting GIT_DIR to $(git rev-parse --git-dir) should not change anything.

Anything else you want to add:

Please review the rest of the bug report below.
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[System Info]
git version:
git version 2.30.1
cpu: x86_64
no commit associated with this build
sizeof-long: 8
sizeof-size_t: 8
shell-path: /bin/sh
uname: Linux 5.3.18-22-default #1 SMP Wed Jun 3 12:16:43 UTC 2020 (720aeba) x86_64
compiler info: gnuc: 9.1
libc info: glibc: 2.26
$SHELL (typically, interactive shell): /opt/local/bin/bash


[Enabled Hooks]