commit-graph: compute Bloom filters for changed paths

commit-graph: compute Bloom filters for changed paths

Add new COMMIT_GRAPH_WRITE_CHANGED_PATHS flag that makes Git compute
Bloom filters for the paths that changed between a commit and it's
first parent, for each commit in the commit-graph.  This computation
is done on a commit-by-commit basis.

We will write these Bloom filters to the commit-graph file, to store
this data on disk, in the next change in this series.

Helped-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Garima Singh <garima.singh@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>