simplify del/reset commands by using readWriteTruncater interface.

remove the type assertions

update the tests:
the tests now check output as well as file contents separately.

cmd/reset_test.go

@@ -2,27 +2,23 @@ package cmd
 
 import (
 	"bytes"
-	"io"
 	"testing"
 )
 
 func TestRunResetCommand(t *testing.T) {
-	var buf bytes.Buffer
+	var out bytes.Buffer
+	var f seekBuffer
 
-	if err := resetAndWriteExcludeFile(&buf, &buf); err != nil {
+	if err := resetAndWriteExcludeFile(&out, &f); err != nil {
 		t.Fatalf("resetAndWriteExcludeFile failed: %v", err)
 	}
 
-	resetContent, err := io.ReadAll(&buf)
-	if err != nil {
-		t.Fatalf("failed to read from temp file: %v", err)
-	}
-
-	if string(resetContent) != defaultExcludeFileContent {
+	if f.String() != defaultExcludeFileContent {
 		t.Errorf(
 			"unexpected content after reset:\nGot:\n%s\nExpected:\n%s",
-			string(resetContent),
+			f.String(),
 			defaultExcludeFileContent,
 		)
 	}
 }
+