BitmapWalkListener: Add method and rename for commits

BitmapWalkListener: Add method and rename for commits

During the walk, the commit can be either
1. already in the walk bitmap
2. unvisited so far with bitmap in the bitmap index
3. unvisited so far without bitmap in the bitmap index

Expose these three states in the interface. This makes the interface
easier to explain: it reports the commits found during the walk.

As it is all about commits, rename the methods to onCommit***.

Change-Id: I661f303eb22d3e735b0e439f16df7ace612376d9