t0027: Add repoMIX and LF_nul

t0027: Add repoMIX and LF_nul

"new safer autocrlf handling":

  - Check if eols in a file are converted at commit, when the file has
    CR (or CRLF) in the repo (technically speaking in the index).
  - Add a test-file repoMIX with mixed line-endings.
  - When converting LF->CRLF or CRLF->LF: check the warnings

checkout_files():

  - Checking out CRLF_nul and checking for eol coversion does not
    make much sense (CRLF will stay CRLF).
  - Use the file LF_nul instead: It is handled a binary in "auto" modes,
    and when declared as text the LF may be replaced with CRLF, depending
    on the configuration.

Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>