t0027: support NATIVE_CRLF platforms

t0027: support NATIVE_CRLF platforms

t0027 expects the native end-of-lines to be a single line feed
character.  On Windows, however, we set it to a carriage return
character followed by a line feed character.  Thus, we have to
modify t0027 to expect different warnings depending on the
end-of-line markers.

Adjust the check of the warnings and use these macros:

  WILC:  Warn if LF becomes CRLF
  WICL:  Warn if CRLF becomes LF
  WAMIX: Mixed line endings: either CRLF->LF or LF->CRLF

Improve the information given by check_warning().

Use test_cmp to show which warning is missing (or shouldn't be
there).

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