CommitGraphWriter: Move path diff calculation to its own class

CommitGraphWriter: Move path diff calculation to its own class

To verify that we have the right paths between commits we are writing
the bloom filters, reading them and querying. The path diff
calculation is tricky enough for correctness and performance that
should be tested on its own.

Move the path diff calculation to its own class, so we can test it on
its own.

This is a noop refactor so we can verify later the steps taken in the
walk.

Change-Id: Ifbdcb752891c4adb08553802f87287de1155bb7c