The new code for properly storing information on a parent file and

fetching it didn't work correctly when the file was created in the

parent diff. get_original_file() was checking if the FileDiff was

new, which would have worked with the older logic because we'd transfer

over the parent revision to the FileDiff. With the new logic, it

didn't know that the file was new, and tried fetching it, failing in the

process.
This change fixes this by checking the parent revision to see if the

file was newly-introduced. If it is, no file is fetched.
There's also a small fix to patching to provide the correct filename.

This is only used for logging purposes and generating files in a patch

error bundle.

Unit tests passed (and verified it fixed some issues found when trying

to merge in to release-4.0.x with some newer tests there).
Successfully posted a change with a file introduced in a parent diff.