fixup handling broken pipes in test transport
fixup handling broken pipes in test transport

Summary:
The test_big.py test triggers this on our internal CI.

This diff fixes up how we detect the EOF on the pipe for CLI transport
that is only used in our tests.

fbshipit-source-id: bd1277d1f0b8408799c22903cd9beed105d4a620

nodakai/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,wez/watchman,facebook/watchman,wez/watchman,kwlzn/watchman,facebook/watchman,nodakai/watchman,wez/watchman,kwlzn/watchman,wez/watchman,kwlzn/watchman,nodakai/watchman,facebook/watchman,kwlzn/watchman,facebook/watchman,wez/watchman,facebook/watchman,wez/watchman,wez/watchman,nodakai/watchman,kwlzn/watchman,facebook/watchman,kwlzn/watchman,facebook/watchman,wez/watchman,nodakai/watchman,wez/watchman,kwlzn/watchman,nodakai/watchman,nodakai/watchman