Add hidden options for tweaking indexing behaviour

[jelmer]

Add hidden options for tweaking indexing behaviour.

[tobixen]

It would probably be prudent to have some kind of test that the script works ... :-)

$ xandikos
Traceback (most recent call last):
  File "/usr/local/bin/xandikos", line 11, in <module>
    load_entry_point('xandikos==0.2.8', 'console_scripts', 'xandikos')()
  File "/usr/local/lib/python3.8/dist-packages/xandikos-0.2.8-py3.8.egg/xandikos/__main__.py", line 29, in main
    return asyncio.run(main(argv))
  File "/usr/lib/python3.8/asyncio/runners.py", line 44, in run
    return loop.run_until_complete(main)
  File "/usr/lib/python3.8/asyncio/base_events.py", line 616, in run_until_complete
    return future.result()
  File "/usr/local/lib/python3.8/dist-packages/xandikos-0.2.8-py3.8.egg/xandikos/web.py", line 1435, in main
    help=argparse.HIDDEN)
AttributeError: module 'argparse' has no attribute 'HIDDEN'