v2writable: preserve timestamps from import

v2writable: preserve timestamps from import

While v2 indexing is triggered immediately after writing the
commit to the git repository, there may be a gap between when
PublicInbox::Import generates a timestamp and when
PublicInbox::SearchIdx sees the message.  So follow the mirror
indexing behavior and take the to-be-indexed (time|date)stamps
directly from the git commit.