vcs: support older versions of git

Fixes golang#51467.

This uses the same flag already used in
`src/cmd/go/internal/modfetch/codehost/git.go`, and avoids using a new
flag that breaks Ubuntu 16 and Centos 7's latest git.

Author: howardjohn

src/cmd/go/internal/vcs/vcs.go

@@ -312,7 +312,7 @@ func gitStatus(vcsGit *Cmd, rootDir string) (Status, error) {
 	// uncommitted files and skip tagging revision / committime.
 	var rev string
 	var commitTime time.Time
-	out, err = vcsGit.runOutputVerboseOnly(rootDir, "show -s --no-show-signature --format=%H:%ct")
+	out, err = vcsGit.runOutputVerboseOnly(rootDir, "-c log.showsignature=false show -s --no-show-signature --format=%H:%ct")
 	if err != nil && !uncommitted {
 		return Status{}, err
 	} else if err == nil {