Summary:
Migrate away from streaming Thrift Python, and instead implement `eden
trace hg` in C++, which will eventually let us support Windows.
This also fixes an unexplained crash in `eden trace hg` on macOS.
Reviewed By: xavierd
Differential Revision: D25515377
fbshipit-source-id: 1916aa7f09df37e9dee3479a858932c724f8aed4
