change request: let's port log begin with start time

[Mark Linimon]

I don't know the internals of tinderbox, but the pointyhat scripts
Just Know about the orders of these headers, so this change should not
be merged back to pointyhat.  (Even changing the scripts would break
the ability to rescan older runs.  Even if we decided that we didn't
care about that, pointyhat would have to be completely idled and the
various builds restarted to do this.)

mcl