Update:
Git for Windows 2.16.1(4) is out and that should fix this issue: https://github.com/git-for-windows/git/releases
Old answer:
This is a known issue in Git for Windows 2.16.1(2) and 2.16.1(3): https://github.com/git-for-windows/git/issues/1481
Only workaround is to use Git for Windows 2.16.1 (Download) (or older; 2.16.0 has other critical issues: TortoiseGit revert failed - unable to revert local changes).
Just for the notes, bugreport in TortoiseGit: https://gitlab.com/tortoisegit/tortoisegit/issues/3156
PS: For Git for Windows >= 2.16 you need at least TortoiseGit 2.5.7 (cf. https://stackoverflow.com/a/48457419/3906760).