t1200: further modernize test script style

t1200: further modernize test script style

Instead of using bare "cmp", use "test_cmp".  Output when the test is run
with a -v option becomes easier to diagnose when something goes wrong
because on saner platforms test_cmp uses "diff -u".

There is no need to put an extra backslash to a line that ends with a '|'
(i.e. the upstream of a pipe).

Signed-off-by: Junio C Hamano <redacted>