t0027: Tests for core.eol=native, eol=lf, eol=crlf

Add test cases for core.eol "native" and "" (unset).
(MINGW uses CRLF, all other systems LF as native line endings)

Add test cases for the attributes "eol=lf" and "eol=crlf"

Other minor changes:
- Use the more portable 'tr' instead of 'od -c' to convert '\n' into 'Q'
  and '\0' into 'N'
- Style fixes for shell functions according to the coding guide lines
- Replace "txtbin" with "attr"

Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Mark t0027-auto-crlf as cheap enough for MinGW

t0027 is marked expensive, but really, for MinGW we want to run these
tests always.

Suggested by Thomas Braun.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge pull request #7 from dscho/mingw-tests

Prepare for more thorough testing when building in MinGW