always require one argument for "reverse"

The "reverse" command had many levels of optional arguments:

	garble [garble flags] reverse [build flags] [package] [files]

This was pretty confusing,
and could easily lead to people running the command incorrectly:

	# note that output.txt isn't a Go package!
	garble reverse output.txt

Moreover, it made the handling of Go build flags pretty confusing.
Should the command below work?

	garble reverse -tags=mytag

It also made it easy to not notice that one must supply the main package
to properly reverse some text that it produced, like a panic message.
With the package path being implicit,
one could mistakenly provide the wrong package by running garble
in a directory containing a different package.

See #394.

main.go

@@ -76,10 +76,12 @@ Similarly, to combine garble flags and Go build flags:
 
 The following commands are supported:
 
-	build [packages]   replace "go build"
-	test [packages]    replace "go test"
-	version            print Garble version
-	reverse [files]    de-obfuscate output such as stack traces
+	build          replace "go build"
+	test           replace "go test"
+	version        print Garble version
+	reverse        de-obfuscate output such as stack traces
+
+To learn more about a command, run "garble help <command>".
 
 garble accepts the following flags before a command:
 
@@ -307,14 +309,19 @@ func mainErr(args []string) error {
 	// If we recognize an argument, we're not running within -toolexec.
 	switch command, args := args[0], args[1:]; command {
 	case "help":
-		if len(args) > 0 {
-			return fmt.Errorf("the help command does not take arguments")
+		if hasHelpFlag(args) || len(args) > 1 {
+			fmt.Fprintf(os.Stderr, "usage: garble help [command]\n")
+			return errJustExit(2)
+		}
+		if len(args) == 1 {
+			return mainErr([]string{args[0], "-h"})
 		}
 		usage()
 		return errJustExit(2)
 	case "version":
-		if len(args) > 0 {
-			return fmt.Errorf("the version command does not take arguments")
+		if hasHelpFlag(args) || len(args) > 0 {
+			fmt.Fprintf(os.Stderr, "usage: garble version\n")
+			return errJustExit(2)
 		}
 		// don't overwrite the version if it was set by -ldflags=-X
 		if info, ok := debug.ReadBuildInfo(); ok && version == "(devel)" {
@@ -1023,8 +1030,10 @@ func processImportCfg(flags []string) (newImportCfg string, _ error) {
 	return newCfg.Name(), nil
 }
 
-type funcFullName = string
-type reflectParameterPosition = int
+type (
+	funcFullName             = string
+	reflectParameterPosition = int
+)
 
 // knownReflectAPIs is a static record of what std APIs use reflection on their
 // parameters, so we can avoid obfuscating types used with them.

reverse.go

@@ -19,24 +19,29 @@ import (
 // commandReverse implements "garble reverse".
 func commandReverse(args []string) error {
 	flags, args := splitFlagsFromArgs(args)
-	if hasHelpFlag(flags) {
-		fmt.Fprintf(os.Stderr, "usage: garble [garble flags] reverse [files]\n")
-		return errJustExit(2)
-	}
+	if hasHelpFlag(flags) || len(args) == 0 {
+		fmt.Fprintf(os.Stderr, `
+usage: garble [garble flags] reverse [build flags] package [files]
+
+For example, after building an obfuscated program as follows:
 
-	mainPkg := "."
-	if len(args) > 0 {
-		mainPkg = args[0]
-		args = args[1:]
+	garble -literals build -tags=mytag ./cmd/mycmd
+
+One can reverse a captured panic stack trace as follows:
+
+	garble -literals reverse -tags=mytag ./cmd/mycmd panic-output.txt
+`[1:])
+		return errJustExit(2)
 	}
 
+	pkg, args := args[0], args[1:]
 	listArgs := []string{
 		"-json",
 		"-deps",
 		"-export",
 	}
 	listArgs = append(listArgs, flags...)
-	listArgs = append(listArgs, mainPkg)
+	listArgs = append(listArgs, pkg)
 	// TODO: We most likely no longer need this "list -toolexec" call, since
 	// we use the original build IDs.
 	if _, err := toolexecCmd("list", listArgs); err != nil {

testdata/scripts/help.txt

@@ -15,7 +15,12 @@ stderr 'garble \[garble flags\] command'
 ! stdout .
 
 ! garble help foo bar
-stderr 'does not take arguments'
+stderr 'usage: garble help'
+! stderr 'Garble obfuscates Go code'
+! stdout .
+
+! garble help -h
+stderr 'usage: garble help'
 ! stdout .
 
 ! garble build -h
@@ -35,7 +40,12 @@ stderr 'Run .go help test.'
 ! stdout .
 
 ! garble reverse -h
-stderr 'garble \[garble flags\] reverse \[files\]'
+stderr 'garble \[garble flags\] reverse \[build flags\] package \[files\]'
+! stderr 'usage: go '
+! stdout .
+
+! garble help reverse
+stderr 'garble \[garble flags\] reverse \[build flags\] package \[files\]'
 ! stderr 'usage: go '
 ! stdout .
 
@@ -59,10 +69,10 @@ garble version
 stdout 'devel|^v0'
 
 ! garble version -flag
-stderr 'does not take arguments'
+stderr 'usage: garble version'
 
 ! garble version arg
-stderr 'does not take arguments'
+stderr 'usage: garble version'
 
 # We need a dummy module for "garble build -badflag".
 -- go.mod --

testdata/scripts/reverse.txt

@@ -1,7 +1,7 @@
 env GOPRIVATE=test/main
 
 # Unknown build flags should result in errors.
-! garble reverse -badflag
+! garble reverse -badflag=foo .
 stderr 'flag provided but not defined'
 
 garble build
@@ -16,7 +16,7 @@ grep 'goroutine 1 \[running\]' main.stderr
 ! grep 'ExportedLib(Type|Field)|unexportedMainFunc|test/main|main\.go|lib\.go' main.stderr
 
 stdin main.stderr
-garble reverse
+garble reverse .
 cmp stdout reverse.stdout
 
 ! garble build ./build-error
@@ -39,12 +39,12 @@ exec ./main
 cp stderr main-literals.stderr
 
 stdin main-literals.stderr
-garble -literals reverse
+garble -literals reverse .
 cmp stdout reverse.stdout
 
 # Reversing a -literals output without the flag should fail.
 stdin main-literals.stderr
-! garble reverse
+! garble reverse .
 cmp stdout main-literals.stderr
 
 -- go.mod --