Inconsistent results from git rev-parse --show-toplevel

["Crabtree, Andrew" <andrew.crabtree@hpe.com>]

I ran into an issue where 'git rev-parse --show-toplevel' is reporting the current directory instead of the root of the repository.
In my case I'm running it from within the pre-commit hook, and it seems like it might be related to GIT_DIR being set. 
It may also be related to using worktrees.  Haven't had a chance to test thoroughly.

Regards,
-Andrew

Git Version -  2.25.0

pre-commit hook

#!/bin/bash
env
pwd
git rev-parse --show-toplevel
cd subdir
pwd
git-rev-parse --show-toplevel

Create a repo with a subdirectory.   Commit as normal.  Output from the hook looks as expected.  Create a worktree, and commit there and it appears as though it is returning incorrect results after changing directory.

Git repo is in /tmp/revparse_test
Worktree is in /tmp/revparse_test_worktree

Committing in /tmp/revparse_test yields the below, as expected 
/tmp/revparse_test
/tmp/revparse_test
/tmp/revparse_test/subdir
/tmp/revparse_test


Committing in the worktree yields below, with the final line seeming incorrect.
/tmp/revparse_test_worktree
/tmp/revparse_test_worktree
/tmp/revparse_test_worktree/subdir
/tmp/revparse_test_worktree/subdir