Add packfile analyzer

Research for #1614 (closed)

Demo in https://youtu.be/34tRcGKqAqU

This MR brings in code I wrote to analyze what exactly is stored in a given packfile bitmap file. It lets you extract two types of information:

• the "object type index": a full coloring of each object in the packfile, indicating its object type (commit, tree, blob or tag)
• reachability information: the list of "indexed commits" and for each of those commits, the list of pack objects reachable from it