Make equalHash more correct

So far it didn't have to handle the case where one hash is empty and the other
isn't, but in the next commit we need that, so let's handle that case correctly.
There's enough logic in the function now that it's worth covering it with tests.

pkg/utils/rebase_todo.go

@@ -56,7 +56,12 @@ func EditRebaseTodo(filePath string, changes []TodoChange, commentChar byte) err
 }
 
 func equalHash(a, b string) bool {
-	return strings.HasPrefix(a, b) || strings.HasPrefix(b, a)
+	if len(a) == 0 && len(b) == 0 {
+		return true
+	}
+
+	commonLength := min(len(a), len(b))
+	return commonLength > 0 && a[:commonLength] == b[:commonLength]
 }
 
 func findTodo(todos []todo.Todo, todoToFind Todo) (int, bool) {