Remove --background flag to prevent `arc` from hanging

Summary:
Ref T4281. A long time ago, we added a `--background` flag to let `arc lint` and `arc unit` run while you're typing a commit message, in some situations.

This code is only moderately beneficial and is way too complicated. Particularly, it has a long history of causing hangs (T4281, T2463), doesn't work on Windows, and is impossible to debug.

It's also running into a serious PHP bug with EAGAIN/EPIPE being indistinguishable that I haven't been able to find a reasonable workaround for in ~3-4 hours of trying.

All the pathways forward that I can see make this already-complex system more complex.

The major reason that this stuff is so complex is that the subprocess may need to prompt the user (notably, to apply patches from lint).

Instead, I'm going to simplify how `arc diff` interacts with `arc lint` and `arc unit`, so we can just fire-and-forget a background process, let it do as much work as it can without needing user input, and then pick up wherever it left off. This will be slightly less cool/magical, but it won't hang bizarrely and I will be able to debug it.

For now, simply remove the `--background` flag and behavior so `arc` works for everyone.

Test Plan: Ran `arc diff` to create this diff.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4281

Differential Revision: https://secure.phabricator.com/D10198