Tweak the --topo/date-order test vector a bit and mark the author
dates of two commits (a2 and a3) earlier than their own committer
dates, making them much older than other commits that are made on
parallel branches to simulate the case where a long running topic
was rebased recently.

They will show up as recent in the --date-order output due to their
timestamps, but they appear a lot later in the --author-date-order
output, even though their committer timestamp says otherwise.

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