--pretty=format: fix broken %ct and %at interpolation

--pretty=format: fix broken %ct and %at interpolation

A pointer arithmetic error in fill_person caused random data
from the commit object to be included with the timestamp,
which looked something like:

    $ git-rev-list --pretty=format:%ct origin/next | head
    commit 98453bdb3db10db26099749bc4f2dc029bed9aa9
    1174977948 -0700

    Merge branch 'master' into next

    * master:
      Bisect: Use
    commit c0ce981f5ebfd02463ff697b2fca52c7a54b0625
    1174889646 -0700

Signed-off-by: Jeff King <redacted>
Acked-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>