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

main_test.go

@@ -622,6 +622,7 @@ func TestRunUsageErrors(t *testing.T) {
 		"report with args":   {[]string{cmdReport, "x"}, usageMarker},
 		"trees with args":    {[]string{cmdTrees, "x"}, usageMarker},
 		"info with args":     {[]string{cmdInfo, "x"}, usageMarker},
+		"dwim with args":     {[]string{cmdDwim, "x"}, usageMarker},
 		"unknown flag":       {[]string{cmdScan, "--nope", "/"}, usageMarker},
 		"unknown subcommand": {[]string{"nope"}, "unknown command"},
 	}