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 the whole thing run without fataling even if a $commit is missing. Future refinements could improve this behavior.

Viewed a file with lines introduced in the initial commit, got empty blame instead of a fatal.