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>
Git 2.0.4

Signed-off-by: Junio C Hamano <gitster@pobox.com>