[BUG] 'git ls-files --no-exclude' segfault & co

'git ls-files' has the options '--exclude', '--exclude-from', '--exclude-per-directory', and '--exclude-standard', which all work fine. However, it also allows the negated version of all these four options, and they definitely don't work very well:

  $ git ls-files --no-exclude
  Segmentation fault
  $ git ls-files --no-exclude-from
  warning: unable to access '(null)': Bad address
  fatal: cannot use (null) as an exclude file

And '--no-exclude-standard' has the same effect as '--exclude-standard', because its parseopt callback function option_parse_exclude_standard() doesn't bother to look at its 'unset' parameter.