Users may set test_cmp to a comparison tool of their liking. The intent is
that the tool performs comparison of line-oriented texts. However, t5300
uses it also to compare binary data. Change those tests to use 'cmp'.
Signed-off-by: Johannes Sixt <j6t@kdbg.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>