child_process stdout being truncated

* **Version**: 9.7.1 * **Platform**: Darwin miless-mbp.lan 17.4.0 Darwin Kernel Version 17.4.0: Sun Dec 17 09:19:54 PST 2017; root:xnu-4570.41.2~1/RELEASE_X86_64 x86_64 * **Subsystem**: It seems like `spawn()` and `exec()` is truncating `stdout`, even when `maxBuffer` is set. I've been trying to capture the output of `jest --help`, which the full output looks like the following: ``` Usage: jest [--config=<pathToConfigFile>] [TestPathPattern] Options: --help, -h Show help [boolean] --all The opposite of `onlyChanged`. If `onlyChanged` is set by default, running jest with `--all` will force Jest to run all tests instead of running only tests related to changed files. --automock Automock all files by default. [boolean] --bail, -b Exit the test suite immediately upon the first failing test. [boolean] --browser Respect the "browser" field in package.json when resolving modules. Some packages export different versions based on whether they are operating in node.js or a browser. [boolean] --cache Whether to use the transform cache. Disable the cache using --no-cache. [boolean] --cacheDirectory The directory where Jest should store its cached dependency information. [string] --changedFilesWithAncestor Runs tests related to the current changes and the changes made in the last commit. Behaves similarly to `--onlyChanged`. [boole…