cmd/go: run git log with --no-show-signature

cmd/go: run git log with --no-show-signature

Git timestamp parsing is broken when fetching modules if the
local git configuration has 'log.showsignature=true'.

Fixes #26388

Change-Id: I47f92381784072335a2a465de56092106c616108
GitHub-Last-Rev: 96f988c0a2dd39a5747ec4a7bad05e7e8ee0d384
GitHub-Pull-Request: golang/go#26389
Reviewed-on: https://go-review.googlesource.com/123958
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>