Add sfdupes dwim: scan / then print duplicate trees (closes #154)

dwim runs what "sfdupes scan /" runs with no flags, then what
"sfdupes trees" runs, and ends with a stderr line saying that
"sfdupes trees" prints the report again. An interrupt or fatal scan
error ends it before the report, as scan ends.

Model: opus-5-5

dwim.go

@@ -0,0 +1,31 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"os"
+	"runtime"
+)
+
+// runDwim implements the dwim subcommand: it runs what "sfdupes scan
+// root" runs with no flags, then what "sfdupes trees" runs, and ends
+// with a line on stderr saying how to print the report again. The dwim
+// command passes "/" as root. An error from the scan, an interrupt
+// included, ends it before the report.
+func runDwim(ctx context.Context, root string, stdout io.Writer) error {
+	err := runScan(ctx, []string{root}, runtime.NumCPU(), false, false)
+	if err != nil {
+		return err
+	}
+
+	err = runTrees(ctx, stdout)
+	if err != nil {
+		return err
+	}
+
+	fmt.Fprintln(os.Stderr,
+		`dwim: run "sfdupes trees" to print this report again`)
+
+	return nil
+}