cmd/go: skip git sha256 tests if git < 2.29

cmd/go: skip git sha256 tests if git < 2.29

Fix test building on older Ubuntu LTS releases (that are still
supported). Git SHA256 support was only included in 2.29, which came out
in 2021. Check the output of `git version` and skip these tests if the
version is older than that introduction.

Thanks to @ianlancetaylor for flagging this.

Updates: #73704
Change-Id: I9d413a63fa43f34f94c274bba7f7b883c80433b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/698835
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Michael Matloob <matloob@google.com>
Auto-Submit: Michael Matloob <matloob@google.com>
Reviewed-by: Ian Alexander <jitsu@google.com>