remote-mediawiki tests: use test_cmp in tests

remote-mediawiki tests: use test_cmp in tests

Change code that used an ad-hoc "diff -b" invocation to use our
test_cmp helper instead. I'm also changing the order of arguments to
be the standard "test_cmp <expected> <actual>".

Using test_cmp has different semantics since the "-b" option to diff
causes it to ignore whitespace, but in these cases the use of "-b" was
just meaningless boilerplate. The desired semantics here are to
compare "git log" lines with know-good data, so we don't want to
ignore whitespace.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>