Add --aggressive option to 'git gc'

Add --aggressive option to 'git gc'

This option causes 'git gc' to more aggressively optimize the
repository at the cost of taking much more wall clock and CPU time.

Today this option causes git-pack-objects to use --no-use-delta
option, and it allows the --window parameter to be set via the
gc.aggressiveWindow configuration parameter.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Junio C Hamano <junkio@cox.net>