Make Windows escaping preserve "%" symbols in arguments

Summary:
Ref T13504. Ref T13209. We currently use "escapeshellarg()" under Windows, which destructively replaces "%" symbols with spaces. This is wrong, breaks general behavior with "git log --format=...", and has led to a lot of weird workarounds in arc code where we don't escape arguments we should be escaping to tiptoe around issues with "%".

Now that execution occurs via "bypass_shell", we can safely (probably?) escape things, although programs are still apparently free to parse the command string however they want.

Test Plan: Added unit tests, ran them under Mac and Windows, got clean results. Ran "arc version" on Mac and Windows.

Maniphest Tasks: T13504, T13209

Differential Revision: https://secure.phabricator.com/D21051