Now that we can access kiln via git, back up the kiln repo locally.

Summary: kiln repos don't have '.git' suffix to them, so we need to add one for dirname.

Test Plan:
Ran it on my machine and it updated webapp fine.

Ran locally, now we don't get

FAILED: timeout 5m git clone --mirror ssh://khanacademy@khanacademy.kilnhg.com/Website/Group/webapp:
fatal: destination path 'webapp.git' already exists and is not an empty directory.

anymore.

Reviewers: benkomalo

Reviewed By: benkomalo

Differential Revision: http://phabricator.khanacademy.org/D2217