Use a persistent child process for git operations

[smashwilson]

As a band-aid for #386 that's achievable by public launch, we could maintain a single, long-living child process à la the ones used byGitPromptServer and use IPC to trigger git operations in that thread instead. That way, our spawn calls at least wouldn't block the event loop for typing events 😡.

We could possibly disable it on Windows or Linux initially depending on whether or not we can ever reproduce it then.

/cc @nathansobo

[nathansobo]

Great to see you take this up. Hate to be a buzzkill harping on this issue, but I'm just terrified of affecting typing performance negatively.