pack-objects: run rev-list equivalent internally.

pack-objects: run rev-list equivalent internally.

Instead of piping the rev-list output from its standard input,
you can say:

	pack-objects --all --unpacked --revs pack

and feed the rev parameters you would otherwise give the
rev-list on its command line from the standard input.
In other words:

	echo 'master..next' | pack-objects --revs pack

and

	rev-list --objects master..next | pack-objects pack

are equivalent.

Signed-off-by: Junio C Hamano <junkio@cox.net>