test_cmp: do not use "diff -u" on platforms that lack one

By default the testsuite calls 'diff -u' whenever a file comparison is called for. Unfortunately that throws a "diff: unknown option '-u'" error for most non-GNU diffs. This patch sets GIT_TEST_CMP to 'cmp' on all the architectures where that happens. The previous version of this patch forgot to export GIT_TEST_CMP from t/Makefile, which is why 'make test' continued to fail most tests on most architectures - test-lib.sh was falling back on its default of `diff -u' for GIT_TEST_CMP. This version of this patch shows a vast improvement in testsuite results where either GNU diff is in the path at configure time, or where Makefile knows that GIT_TEST_CMP=cmp is required. Signed-off-by: Gary V. Vaughan <gary@thewrittenword.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>