Add "aggressive" option to GC

Add "aggressive" option to GC

JGit should offer the possibility to do a garbage collection in
"aggressive" mode. In this mode garbage collection more aggressively
optimize the repository at the expense of taking much more time.
Technically a aggressive mode garbage collection differs from a
non-aggressive one by:
- not reusing packed objects found in old packs. Recompress every object
- the configuration pack.window is set to 250 (the default is 10)
- the configuration pack.depths is set to 250 (the default is 50)

The associated classes in org.eclipse.jgit.api and the command line
command in org.eclipse.jgit.pgm expose this new option.

The configuration parameters gc.aggressiveDepth and gc.aggressiveWindow
have been introduced to configure this feature.

Bug: 444332
Change-Id: I024101f2810acf6be13ce144c9893d98f5c4ae76