Stop "git blame" from printing "^" markers on root repository commits

Summary: Depends on D19391. Ref T13126. See that task for some details on what's going on here.

Test Plan:

• Viewed a file which includes lines that were added during the first commit to the repository.
• Before D19391: fatal.
• After D19391: blank.
• After this patch: accurate blame information.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13126

Differential Revision: https://secure.phabricator.com/D19392