ds/commit-graph-incremental (was Re: What's cooking in git.git (Jun 2019, #04; Fri, 14))

> * ds/commit-graph-incremental (2019-06-12) 16 commits
>  - commit-graph: test --split across alternate without --split
>  - commit-graph: test octopus merges with --split
>  - commit-graph: clean up chains after flattened write
>  - commit-graph: verify chains with --shallow mode
>  - commit-graph: create options for split files
>  - commit-graph: expire commit-graph files
>  - commit-graph: allow cross-alternate chains
>  - commit-graph: merge commit-graph chains
>  - commit-graph: add --split option to builtin
>  - commit-graph: write commit-graph chains
>  - commit-graph: rearrange chunk count logic
>  - commit-graph: add base graphs chunk
>  - commit-graph: load commit-graph chains
>  - commit-graph: rename commit_compare to oid_compare
>  - commit-graph: prepare for commit-graph chains
>  - commit-graph: document commit-graph chains
>  (this branch uses ds/commit-graph-write-refactor; is tangled with ds/close-object-store.)
> 
>  The commits in a repository can be described by multiple
>  commit-graph files now, which allows the commit-graph files to be
>  updated incrementally.
> 
>  Will merge to 'next'.