Cannot stage a folder which contains a file with name ".git"

Reproduction steps

1. Use LibGit2Sharp to work on a local git repository
2. Create some random sub folders in that local repository folder
3. Create in some of these subfolders a file with the name ".git"
4. Try to stage all changes

Expected behavior

Like with e.g. Git for Windows
https://git-scm.com/download/win
The file gets just added to the repro without any exception

At least I would expect a better error description

Actual behavior

The LibGit2Sharp stage process crashes with an exception.
The exception contains the message "Invalid path"
It took a while till I figured out why this actually happens.

Version of LibGit2Sharp (release number or SHA1)

Nuget version of LibGit2Sharp: 0.26.0 (Current latest)
Nuget version of LibGit2Sharp.NativeBinaries: 2.0.267 (Newer version available but not compatible to newest LibGit2Sharp)

Operating system(s) tested; .NET runtime tested

Windows 10 Version 1903 - Build 18362.239

Project settings

The project in which I use the LibGit2Sharp is build with Visual Studio 2019.
The project uses the .Net Framework 4.7.2

サブフォルダーに ".git" という名前のファイルを含むリポジトリを使用して、LibGit2Sharp 0.26.0 および LibGit2Sharp.NativeBinaries 2.0.267 で Windows 10 上の staging を再現する。Git for Windows との動作を比較し、その後、"Invalid path" 例外を発生させる staging パスを追跡する。ファイルを例外なしで staging できるか、またはエラーが原因を明確に示せれば完了とする。