Windows Explorer context menu: TortoiseGit doesn't recognize local Git working copy if "gitdir-redirection" is used within same folder

What steps will reproduce the problem?

1. Use a non-standard git folder, say "_git\" instead of ".git\" (you may rename an existing ".git\").
2. Create a text file ".git" with "gitdir: _git" to direct git to the non-standard git folder.
3. Open the Explorer context menu to commit changes.
4. Context menu contains "Git Clone", "Git Create ..." etc.

What is the expected output? What do you see instead?

The context menu should contain "Git Sync ...", "Git Commit ..." etc.

What version of TortoiseGit and Git are you using? On what operating system?

The bug occured with TortoiseGit 2.8.0, Git 2.21.0.windows.1, under Windows 10 Pro 1809 (17763.316, German localization).

Please provide any additional information below.

TortoiseGit 2.7.0 and earlier work as expected.

I actually have no return/newline after "gitdir: _git".