Fix crash on diff of new empty file
Fix crash on diff of new empty file

When adding (or presumably removing) an empty file, there is no
diff header, but that's where Gertty was getting the filenames.
Instead, get them from the git blob itself.

Change-Id: Ib2908aaf1d7554df53b5fb262017d85f3a560066
Story: 161

openstack/gertty,stackforge/gertty,aspiers/gertty