PackWriter: support excluding objects already in other packs

PackWriter: support excluding objects already in other packs

This can be useful when implementing garbage collection and there
are packs that should not be copied, such as huge packs that have
a sibling ".keep" file alongside of them.

Callers driving PackWriter need to initialize the list of packs not
to include objects from by passing each index to excludeObjects().

Change-Id: Id7f34df69df97be406bcae184308e92b0e8690fd
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Chris Aniszczyk <caniszczyk@gmail.com>