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/123958Reviewed-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>

src/cmd/go/internal/modfetch/codehost/git.go

@@ -362,7 +362,7 @@ func (r *gitRepo) stat(rev string) (*RevInfo, error) {
 // statLocal returns a RevInfo describing rev in the local git repository.
 // It uses version as info.Version.
 func (r *gitRepo) statLocal(version, rev string) (*RevInfo, error) {
-	out, err := Run(r.dir, "git", "log", "-n1", "--format=format:%H %ct %D", rev)
+	out, err := Run(r.dir, "git", "log", "-n1", "--format=format:%H %ct %D", "--no-show-signature", rev)
 	if err != nil {
 		return nil, fmt.Errorf("unknown revision %s", rev)
 	}