PackWriter: Cleanup findObjectToPack method

PackWriter: Cleanup findObjectToPack method

Some of this code predates making ObjectId.equals() final
and fixing RevObject.equals() to match ObjectId.equals().
It was therefore more complex than it needs to be, because
it tried to work around RevObject's broken equals() rules
by converting to ObjectId in a different collection.

Also combine setUpWalker() and findObjectsToPack() methods,
these can be one method and the code is actually cleaner.

Change-Id: I0f4cf9997cd66d8b6e7f80873979ef1439e507fe
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Chris Aniszczyk <caniszczyk@gmail.com>