Repository state machine

[smashwilson]

Don't worry, this doesn't really have 75+ commits, it just builds on top of #633.

Refactor the Repository model into a state machine. This will fix #608 by allowing us to render distinct things depending on more granular information than this.props.repository === null and simplify a bunch of code because we can ensure we always have a non-null Repository instance available.

Here's the state hierarchy:

• State
  • Undetermined
  • Absent
  • Loading
  • Initializing
  • Cloning
  • Empty
  • Present
  • Destroyed

Countdown to Blastoff 🚀

• Introduce the State class hierarchy.
  • Implement methods on State that provide default behavior for visible methods: returning empty collections or null objects for accessors, throwing an Error for action methods.
  • Mark State methods intended to be used from a Repository with a @shouldDelegate decorator.
  • Selectively override methods in each State subclass. Most of Repository moves to Present.
  • Allow State subclasses to transition to one another by constructor name (to avoid circular dependencies).
  • Export only the default state (Loading) to encapsulate the states.
  • Introduce an Absent state to represent a missing working directory.
• Incorporate state management and delegation into Repository.
  • Synchronously construct Repository instances in their initial state.
  • Provide methods to transition among states.
  • Emit an event when the current state changes.
  • Create a Promise that can be used to wait for a loading repository to complete.
  • Delegate a bunch of stuff to this.state.
  • Add a test case to ensure that the delegated methods are up to date with the @shouldDelegate decorations.
  • Adjust the rest of the Repository tests to accommodate the state transition model.
• Introduce model objects for Branch, Remote, and Commit.
  • Create models and null objects for each.
  • Return model or null instances from appropriate methods in State and Present.
  • Propagate changes to those methods' callers.
  • Fix up PropTypes.
  • Move test for githubInfoFromRemote.
• Use the Repository constructor in WorkdirContext.
• Mark repository as .isRequired in our PropTypes.
• Render GitTabController differently depending on the current repository state.
  • The "click here to initialize" view should be shown when isEmpty().
  • Maybe just grey it all out when isLoading().
  • Show the existing view when isPresent().
• Introduce an Undetermined state that's used when activated without any projects in the workspace. It should automatically transition to Absent after a delay if no projects are loaded.
• Render StatusBarTileController differently depending on the current repository state.
• Prompt for and create a working directory when performing init or clone from the Absent state.
  • This should be the case with an active TextEditor on a file that isn't within a git repository, too.
• Change the init verbiage in GitTabView depending on whether or not you have an existing directory to init.
• Use atom.getLoadSettings() to determine whether the Undetermined state acts like a Loading state or an Absent one.
• Document the states and provide a state transition diagram.
• Handle the deletion of the final project directory