ls-files: unbreak "ls-files -i"

ls-files: unbreak "ls-files -i"

Commit b5227d8 changed the behavior of "ls-files" with
respect to includes, but accidentally broke the "-i" option
The original behavior was:

  1. if no "-i" is given, cull all results according to --exclude*
  2. if "-i" is given, show the inverse of (1)

The broken behavior was:

  1. if no "-i" is given:
     a. for "-o", cull results according to --exclude*
     b. for index files, always show all
  2. if "-i" is given:
     a. for "-o", shows the inverse of (1a)
     b. for index files, always show all

The fixed behavior keeps the new (1b) behavior introduced
by b5227d8, but fixes the (2b) behavior to show only ignored
files, not all files.

This patch also tweaks the documentation. The original text
was somewhat obscure in the first place, but it is also now
inaccurate (the relationship between (1b) and (2b) is not
quite a "reverse").

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>