t3436: check --committer-date-is-author-date result more carefully

t3436: check --committer-date-is-author-date result more carefully

After running "rebase --committer-date-is-author-date", we confirm that
the committer date is the same as the author date. However, we don't
look at any other parts of the committer ident line to make sure we
didn't screw them up. And indeed, there are a few bugs here. Depending
on the rebase backend in use, we may accidentally use the author email
instead of the committer's, or even an empty string.

Let's teach our test_ctime_is_atime helper to check the committer name
and email, which reveals several failing tests.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>