Print help that was asked for to stdout (closes #179)

--help, -h and the help subcommand now print to stdout and exit 0, so
the help can be paged and searched. The usage text of a usage error
stays on stderr with exit 2, as does help for an unknown subcommand
("sfdupes help scna"). A failed write of the help exits 1.

Cobra prints help to the command's output writer, which stays stderr,
drops a failed write, and returns no error after help. So run replaces
the help function and the help subcommand: the help goes into a buffer,
then to stdout, and the error sets the exit code.

README "Settings" and goal 4 say so.

Model: opus-5-5

README.md

@@ -253,9 +253,9 @@ In order:
    `trees` check only that each copy they name still exists, and leave out a
    copy that is gone. `scan` is built to run from `cron`. The reports run at any
    time, against what the database holds.
-4. **Keep the streams apart.** Everything on stdout is machine-readable data.
-   Everything for a human reader, such as progress and warnings, goes to stderr.
-   The one exception is help that was asked for, which goes to stdout.
+4. **Keep the streams apart.** stdout holds only what was asked for:
+   machine-readable data, or help. Everything else for a human reader, such as
+   progress and warnings, goes to stderr.
 
 ### Subcommands
 

internal/sfdupes/cli_test.go

@@ -773,6 +773,36 @@ func TestRunHelp(t *testing.T) {
 	}
 }
 
+func TestRunHelpScanPrintsScanHelp(t *testing.T) {
+	t.Parallel()
+
+	// "sfdupes help scan" and "sfdupes --help scan" print exactly what
+	// "sfdupes scan --help" prints: the help of scan, not of sfdupes.
+	var want bytes.Buffer
+
+	run([]string{cmdScan, helpFlag}, &want, io.Discard)
+
+	for _, args := range [][]string{
+		{helpCommand, cmdScan}, {helpFlag, cmdScan},
+	} {
+		var stdout, stderr bytes.Buffer
+
+		code := run(args, &stdout, &stderr)
+		if code != exitOK {
+			t.Errorf("run(%v) = %d, want %d", args, code, exitOK)
+		}
+
+		if got := stdout.String(); got != want.String() {
+			t.Errorf("run(%v) stdout = %q, want what scan %s prints, %q",
+				args, got, helpFlag, want.String())
+		}
+
+		if got := stderr.String(); got != "" {
+			t.Errorf("run(%v) stderr = %q, want nothing", args, got)
+		}
+	}
+}
+
 func TestRunScanUsageLine(t *testing.T) {
 	t.Parallel()