Merge branch 'ds/commit-graph'

Precompute and store information necessary for ancestry traversal in a separate file to optimize graph walking. * ds/commit-graph: commit-graph: implement "--append" option commit-graph: build graph from starting commits commit-graph: read only from specific pack-indexes commit: integrate commit graph with commit parsing commit-graph: close under reachability commit-graph: add core.commitGraph setting commit-graph: implement git commit-graph read commit-graph: implement git-commit-graph write commit-graph: implement write_commit_graph() commit-graph: create git-commit-graph builtin graph: add commit graph design document commit-graph: add format document csum-file: refactor finalize_hashfile() method csum-file: rename hashclose() to finalize_hashfile()