t5000, t5003: do not use test_cmp to compare binary files

test_cmp() is primarily meant to compare text files (and display the
difference for debug purposes).

Raw "cmp" is better suited to compare binary files (tar, zip, etc.).

On MinGW, test_cmp is a shell function mingw_test_cmp that tries to
read both files into environment, stripping CR characters (introduced
in commit 4d715ac0).

This function usually speeds things up, as fork is extremly slow on
Windows.  But no wonder that this function is extremely slow and
sometimes even crashes when comparing large tar or zip files.

Signed-off-by: Stepan Kasal <kasal@ucw.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge pull request #222 from t-b/backport-test_cmp_bin

Backport of ae0cb2 from master