Make time-based commit filtering work with topological ordering.

Make time-based commit filtering work with topological ordering.

The trick is to consider the time-based filtering a limiter, the same way
we do for release ranges.

That means that the time-based filtering runs _before_ the topological
sorting, which makes it meaningful again. It also simplifies the code
logic.

This makes "gitk" useful with time ranges.

[ Second version: --merge-order now unaffected by the re-org ]

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>