Add scan --exclude PATH (closes #163) (#165)

internal/sfdupes/scan_test.go

@@ -2289,7 +2289,7 @@ func TestPruneRoots(t *testing.T) {
 
 	// The walk of /p does not enter /p/q when it is a default exclude, so
 	// /p/q is kept, without -x too; /p/q/r lies under it and is dropped.
-	got = pruneRoots(nested, false, walkRules{excludes: []string{pqRoot}},
+	got = pruneRoots(nested, false, walkRules{defaultExcludes: []string{pqRoot}},
 		device)
 	if !slices.Equal(got, want) {
 		t.Fatalf("pruneRoots excluding %s = %q, want %q", pqRoot, got, want)