tests: use "test_cmp", not "diff", when verifying the result

In tests, call test_cmp rather than raw diff where possible (i.e. if
the output does not go to a pipe), to allow the use of, say, 'cmp'
when the default 'diff -u' is not compatible with a vendor diff.

When that is not possible, use $DIFF, as set in GIT-BUILD-OPTIONS.

Signed-off-by: Gary V. Vaughan <gary@thewrittenword.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>