nicer eye candies for pack-objects

nicer eye candies for pack-objects

This provides a stable and simpler progress reporting mechanism that
updates progress as often as possible but accurately not updating more
than once a second.  The deltification phase is also made more
interesting to watch (since repacking a big repository and only seeing a
dot appear once every many seconds is rather boring and doesn't provide
much food for anticipation).

Signed-off-by: Nicolas Pitre <redacted>
Signed-off-by: Junio C Hamano <redacted>