t7600 (merge): modernize style

t7600 (merge): modernize style

Guard setup commands with test_expect_success, so they are easier
to visually skip over and get to the good part.  While at it:

 - use "printf '%s\n' a b ..." instead of "cat <<EOF" for test
   vectors with short lines;

 - use test_cmp instead of test foo = bar where possible, for
   better output with -v on failure;

 - do not go to extraordinary lengths to print a relevant message
   when test commands fail.  There is a patch in flight that could be
   used to restore the nice error messages in a cleaner way.

Cc: Lars Hjemli <hjemli@gmail.com>
Cc: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>