Use the C.UTF-8 locale for executing subprocesses.

A while back, we changed our environment for executing subprocesses from
en_US.UTF-8 to C, because not everyone has a US English locale installed on
their machine. This broke subversion diffs when filenames or the diffs
contained unicode characters. Using the C.UTF-8 locale fixes this.

Testing done:
Had someone who was encountering this issue try the code change, and they
reported that it's fixed.

Reviewed at https://reviews.reviewboard.org/r/5752/