Don't choke on copy-pasted diffs which include commit messages

Summary: If you git show and copy/paste it into Differential, we die trying to save the DifferentialChangeset corresponding to the commit message (error: column "filename" can not be null). Instead, drop the message change for raw diffs.

Test Plan: Copy/pasted git show into Differential.

Reviewers: btrahan, vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D3740