Project save backed by git

[hubertp]

Pull Request Description

This change adds support for Version Controlled projects in language server.
Version Control supports operations:

• init - initialize VCS for a project
• save - commit all changes to the project in VCS
• restore - ability to restore project to some past save
• status - show the status of the project from VCS' perspective
• list - show a list of requested saves

Important Notes

Behind the scenes, Enso's VCS uses git (or rather jGit) but nothing stops us from using a different implementation as long as it conforms to the establish API.

Checklist

Please include the following checklist in your PR:

• The documentation has been updated if necessary.
• All code conforms to the
  Scala,
  Java,
  and
  Rust
  style guides.
• All code has been tested:
  • Unit tests have been written where possible.
  • If GUI codebase was changed: Enso GUI was tested when built using BOTH
    ./run ide build and ./run ide watch.

[JaroslavTulach]

Looks like a standard PR for language server. Two things I find strange:

• Akka is a an actor framework, there should be no blocking waiting - but there is. Why?
• JGit is added as a dependency of the whole language-server project - that is not modular enough - we should have more compilation units and then just assemble them in runtime.

None of my comments is critical. Hence approving anyway.

[JaroslavTulach]

Shouldn't this whole VCS related functionality be in its own module? Something like engine/language-server-vcs? It looks like a quite independent sub-system with its own dependency needs. Separating it into an independent compilation unit would prevent other parts of language-server code to use JGit API directly when it is supposed to be hidden.

Option1: Define the VcsApi in 'language-servermodule without any dependencies on JGit. Implement it in thelanguage-server-vcsimplmodule that will have dependency on JGit as well aslanguage-server` module. Use ServiceLoader to connect them together.

Option2: Define the VcsApi in a separate module language-server-vcs. Implement it in language-server-vcsimpl module that will have dependency on JGit as well as language-server-vcs module. Change the language-server module to depend on language-server-vcs, use its API. The API module would use ServiceLoader again to find the right implementation during runtime.

[hubertp]

Seemed like a bit of an overkill at this point. Mostly because we don't use any other VCS implementation right now. If we had more implementations, I would totally agree with you.
Similar thing can be said about other elements of the language server like file manager and all buffer-related stuff.

[JaroslavTulach]

Similar thing can be said about other elements of the language server

Yes. There are many other violations, but when will we start fixing them if not when introducing completely new library? Or will we continue building such a megalitic monolith forever?

[hubertp]

That sounds to me more like a regular refactoring action that could be applied to different subprojects of language-server. Mixing that work with project save done here, would make review rather difficult.

So how about we schedule that work with @4e6 separately?

[hubertp]

Created https://www.pivotaltracker.com/story/show/183794226 as a follow up work.

[4e6]

Love the fact that you removed the restriction on unique commit name ❤️

Approved, just don't forget to update the protocol-language-server.md API doc with the new VCS methods before merging.