cmd/go/internal/modfetch/codehost: fix git code export on Windows
cmd/go/internal/modfetch/codehost: fix git code export on Windows

Git was helpfully putting \r\n line endings into the files during "git archive",
which meant that the hash validation didn't work. Fix that.

I believe this bug would have made TestModVerify fail on a
Windows system with git installed and with the default git settings,
but I don't have such a system easily available to test with.

Thanks to CJ-Jackson on GitHub for identifying the right Git flags.

Fixes golang/go#26229.

Change-Id: Ic33ef7272eb3a5cc6b43c361a7d3a1b1d55db714
Reviewed-on: https://go-review.googlesource.com/122480
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

golang/vgo,golang/vgo