R1:95e179d9a4f3

Fix a fatal in the document engine blame view with files that blame to the initial commit

Summary:
Ref T13126. When you view a file using the new document engine view and some lines were introduced in the initial commit to the repository, Git renders "^abc123" in the blame output.

We currently don't do anything about this, and later fail to look it up and fatal.

It's also unlikely-but-conceivably-possible to end up here if a commit has not imported yet or has been nuked with `bin/remove destroy`.

Let…