Allow `arc.bat` to work from paths with spaces in them

Summary: If you dump `arcanist/` into "Program Files" or similar, the `.bat` currently breaks. Quote the path so it doesn't. Fixes T1436.

Test Plan: Ran `arc` with `arcanist/` in a path with spaces in it.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1436

Differential Revision: https://secure.phabricator.com/D4375