def METHOD_NAME(self) -> None:
    """Process a diff header line."""
    if self.line.startswith('Binary files '):
        self.file.binary = True
    elif self.line == '--- /dev/null':
        self.file.old.exists = False
    elif self.line == '+++ /dev/null':
        self.file.new.exists = False
    else:
        self.file.append_header(self.line)