Augment existing data model w/ learnings from github.com/community-graph

- [github.com/community-graph](http://github.com/community-graph)
- [github.com/community-graph/documentation](http://github.com/community-graph/documentation)

### Here's the model we have today:


### ...and how it relates to the community-graph's model:



### Here's the community-graph model:

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Note that the community graph data import uses the GH GraphQL api. In this project (landscape-graph) the bulk of git info will be coming from src-d/gitbase, however having the interactive / GraphQL mechanism is also useful.

* The git model is already quite close, however the current landscape-graph model doesn't model Issues, and it probably should.

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- [ ] Add Issue to the Git Model
- [ ] Determine what else we should take (now), and what to do in the future (new issues)