Use a temporary file to execute `arc patch`

Summary:
Piping data around on Windows doesn't work well when it contains zany characters like "null" and "newline". Fixes T3266.

Instead of piping data into `git apply`, write to a temporary file.

Test Plan:
Ran `arc patch`, got good results.

  >>> [17] <exec> $ git apply --index --reject -- '/private/var/folders/8k/c3vkmjy5335gcxdzxkhwq82w0000gn/T/7z9iea6srikoo0sc/4266-ZEyvz9'

Reviewers: btrahan, hach-que

Reviewed By: hach-que

CC: aran

Maniphest Tasks: T3266

Differential Revision: https://secure.phabricator.com/D6070