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

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

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13126

Differential Revision: https://secure.phabricator.com/D19391