Git Extensions "function not implemented"

The new releases, Git for Windows 2.16.1(3) and Git for Windows 2.16.1(4), fix this regression:

2.16.1(3) Bug Fixes

• When http.sslBackend is not configured (e.g. in portable Git or MinGit), fetch/push operations no longer crash.
• On Windows 7 and older, Git for Windows v2.16.1(2) was no longer able to spawn any processes (e.g. during fetch/clone). This regression has been fixed.
• The Perl upgrade in v2.16.1(2) broke git send-email; This has been fixed by updating the Net-SSLeay Perl module.

The mentioned regression in the second bullet point is exactly the error this question is about.

Unfortunately, there was one missing issue about spawning processes which was then fixed in the subsequent fourth release:

2.16.1(4) Bug Fixes

• When called from TortoiseGit, git.exe can now spawn processes again.

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Note that the download on https://git-scm.com/download/win might not have been update yet, so you need to download it from gitforwindows.org or directly from the release page on GitHub.

Afterwards, you can verify that you indeed running the new version using git --version which should report git version 2.16.1.windows.4.

Per Philippe and Jake - bug is filed against GIT 2.16.2, and workaround is to go back to prior version.

https://github.com/git-for-windows/git/releases/tag/v2.16.1.windows.1