cmd/go/internal/modfetch/codehost: set core.longpaths in Git repos on Windows

cmd/go/internal/modfetch/codehost: set core.longpaths in Git repos on Windows

This setting appears to be needed to avoid “Filename too long” errors
when downloading modules from repos with long branch names,
particularly if the path to the module cache is already fairly long
(as may be the case in CI systems and in tests of cmd/go itself).

Change-Id: I3aa89ea872b29eb0460c8a8afc94f182a68982fd
Reviewed-on: https://go-review.googlesource.com/c/go/+/482819
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>