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

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>