[release-branch.go1.21] cmd/go: missing name in failed command error

Fixed the error reporting for an unknown command to
preserve the name when displaying the error message.

Fixes #61604

Change-Id: I13defb84e61265ab48ab514e9d4f1626a4a3f758
GitHub-Last-Rev: 5d2889c
GitHub-Pull-Request: #61607
Reviewed-on: https://go-review.googlesource.com/c/go/+/513555
Run-TryBot: Russ Cox <[email protected]>
Reviewed-by: David Chase <[email protected]>
Auto-Submit: Russ Cox <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Russ Cox <[email protected]>
Reviewed-on: https://go-review.googlesource.com/c/go/+/515278
Auto-Submit: David Chase <[email protected]>

Author: mauri870

src/cmd/go/main.go

@@ -175,7 +175,11 @@ func main() {
 		if used > 0 {
 			helpArg += " " + strings.Join(args[:used], " ")
 		}
-		fmt.Fprintf(os.Stderr, "go %s: unknown command\nRun 'go help%s' for usage.\n", cfg.CmdName, helpArg)
+		cmdName := cfg.CmdName
+		if cmdName == "" {
+			cmdName = args[0]
+		}
+		fmt.Fprintf(os.Stderr, "go %s: unknown command\nRun 'go help%s' for usage.\n", cmdName, helpArg)
 		base.SetExitStatus(2)
 		base.Exit()
 	}

src/cmd/go/testdata/script/go_badcmd.txt

@@ -0,0 +1,2 @@
+! go asdf
+stderr '^go asdf: unknown command'