Revert "Merge pull request #42376 from dtzWill/update/git-2.18.0"

This reverts commit 2624f907, reversing
changes made to 37aab4cb.

See:
https://github.com/NixOS/nixpkgs/pull/42376#issuecomment-399789096

Basically this breaks at least some users of fetchgit,
so let's revert this until this is sorted out.