Custom repository contexts: show, hide, complete, quit and multi-context projects

[mostafaeweda]

This extends #222 to support custom repository (hg in that case) detection of going into and out of rebase/merge conflict state and requesting to show or hide the merge conflicts UI accordingly.

These changes support multiple context providers & activating for the one with the highest priority that is indeed in a conflicting state.

Finally, it changes the complete & quit to be context-specific so that can be defined from Nuclide side to support automation of running the appropriate commands and remote development.

callback-to-promise and some minor simplifications are along the lines.

I'll be sharing the flow types defined for merge-conflicts APIs when I finish my changes on the nuclide side.

[smashwilson]

👍

[mostafaeweda]

Can I get a version bump release to have this be in apm install merge-conflicts?

[smashwilson]

Done:

~/code/atom/merge-conflicts (master=) 
$ apm publish patch
Preparing and tagging a new version ✓
Pushing v1.4.4 tag ✓
Publishing merge-conflicts@v1.4.4 ✓

By the way, I believe that you can do apm publish too because you're a collaborator on this repo. No need to wait for me if I'm not around 😄

The only extra release work I do is (a) try to bring the CHANGELOG up to date with the work that's about to and (b) copy and paste the latest CHANGELOG entry into a new release for the tag that gets pushed.

[mostafaeweda]

Great, will do that next time 👍