Runtime pack access is done in the pack file mtime order since recent
packs are more likely to contain frequently used objects than old packs.
However the --max-pack-size option can produce multiple packs with mtime
in the reversed order as newer objects are always written first.

Let's modify mtime of later pack files (when any) so they appear older
than preceding ones when a repack creates multiple packs.
Signed-off-by: NNicolas Pitre <nico@cam.org>