Optimize DFS object reuse selection code

Optimize DFS object reuse selection code

Rewrite this complicated logic to examine each pack file exactly
once. This reduces thrashing when there are many large pack files
present and the reader needs to locate each object's header.

The intermediate temporary list is now smaller, it is bounded to
the same length as the input object list. In the prior version of
this code the list contained one entry for every representation of
every object being packed.

Only one representation object is allocated, reducing the overall
memory footprint to be approximately one reference per object found
in the current pack file (the pointer in the BlockList). This saves
considerable working set memory compared to the prior version that
made and held onto a new representation for every ObjectToPack.

Change-Id: I2c1f18cd6755643ac4c2cf1f23b5464ca9d91b22