In Arcanist, when trying to write to a file configuration source, create missing directories
Summary: The ".git/arc" directory may need to be created when writing to working copy configuration.
Test Plan:
- Tried to store an answer to a prompt in a new working copy.
- Before: error that ".git/arc" does not exist.
- After: prompt saved to working copy configuration.
Differential Revision: https://secure.phabricator.com/D21588