The old version git not support "--no-show-signature", git add this from
v2.10.0.

Fixes golang/go#26501.

Change-Id: Ia6b54488651e8687b08a4d40e092822bf960c4fe
Reviewed-on: https://go-review.googlesource.com/125315
Run-TryBot: Baokun Lee <nototon@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>