paint_down_to_common(): parse commit before relying on its timestamp

paint_down_to_common(): parse commit before relying on its timestamp

When refactoring the merge-base computation to reduce the pairwise
O(n*(n-1)) traversals to parallel O(n) traversals, the code forgot
that timestamp based heuristics needs each commit to have been
parsed.  This caused an empty "git pull" to spend cycles, traversing
the history all the way down to 0 (because an unparsed commit object
has 0 timestamp, and any other commit object with positive timestamp
will be processed for its parents, all getting parsed), only to come
up with a merge message to be used.

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