Revert the changes to the subprocess locales.

We originally used 'en_US.UTF-8' as the locale for executing subprocesses.
After a bug report from someone who didn't have the en_US locale on their
system, we changed it to 'C', but that broke non-ASCII commit messages with
SVN. The third try, 'C.UTF-8' isn't well supported (yet). Overall, it seems
like our original choice, 'en_US.UTF-8' is the best (least worst) of these.

Ideally this would be override-able in the config file, but the execute()
method doesn't yet have the plumbing to do this. I've added a comment to this
effect.

Testing done: Ran unit tests.

Reviewed at https://reviews.reviewboard.org/r/5772/