commit --amend: test specifies authorship but forgets to check

The test case "--amend option copies authorship" specifies that the
git-commit option `--amend` uses the authorship of the replaced
commit for the new commit. Add the omitted check that this property
actually holds.

Signed-off-by: Fabian Ruch <bafain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: bafain

t/t7509-commit.sh

@@ -77,6 +77,7 @@ test_expect_success '--amend option copies authorship' '
 	git commit -a --amend -m "amend test" &&
 	author_header Initial >expect &&
 	author_header HEAD >actual &&
+	test_cmp expect actual &&
 
 	echo "amend test" >expect &&
 	message_body HEAD >actual &&