Minimal requirements to support `hit-on`

[chshersh]

The first project to use github-graphql is going to be hit-on. And for this to happen, we need to support the following types and operations:

• [x] Query
  • [x] Issue return fields
    • [x] Issue number
    • [x] Title
    • [x] Author
    • [x] Assignees
    • [x] Labels
    • [x] URL
    • [x] Description
    • [x] Closed
  • [x] Milestone return fields
    • [x] Id
    • [x] Number (for sorting and taking the latest)
    • [x] Name
    • [x] Number of open issues
    • [x] Number of closed issues
  • [x] Get all issues and filter by the following fields
    • [x] Owner
    • [x] Repository
    • [x] Open state
  • [x] Get a single issue by number
  • [x] Get all pull requests
    • [x] Owner
    • [x] Repo
    • [x] Option (only single PR per branch is allowed): let headPrMod = optionsHead $ untagName owner <> ":" <> curBranch
  • [x] RepositoryId (needed for many mutation queries)
    • [x] Owner
    • [x] Name
• [x] Mutation
  • [x] Create new issue
    • [x] RepositoryId
    • [x] Title
    • [x] Optional milestone
  • [x] Assign issue
    • [x] By number/id
    • [x] Author

Since GitHub GraphQL API provides more flexibility than the REST API, we probably can improve some of our queries in hit-on by fetching less or by filtering directly in the query. So, other possible suggestions and improvements are welcome 🙂