R1:7e6bae471c52

Always setlocale() to en_US.UTF-8 for the main process

Summary:
Depends on D18987. See PHI343. Fixes T13060. See also T7339.

When the main process starts up with `LANG=POSIX` (this is the default on Ubuntu) and we later try to run a subprocess with a UTF8 character in the argument list (like `git cat-file blob 🐑.txt`), the argument is not passed to the subprocess correctly.

We already set `LANG=en_US.UTF-8` in the //subprocess// environment, but this only controls behavior for the subprocess itself. It…