We can still race and read the Commit before it has CommitData. For most other objects, we'd fail the load and end up in trouble again.

However, in the special case of Commit, if we ask for CommitData and it doesn't exist yet, we get a dummy/empty one instead of a "you didn't ask for commit data but are trying to use it now" exception, specifically because stuff can race the creation of CommitData.

I didn't actually verify that an empty/default CommitData doesn't hit some other error, but from inspection it looks like it will be okay (we'll just end up with a null authorPHID, which the code should probably handle anyway).

Specifically, in CommitQuery near line 400, we do this:

if (!$commit_data) {
  $commit_data = new PhabricatorRepositoryCommitData();
}

In most other applications, that would be:

if (!$commit_data) {
  $this->didRejectResult($commit);
  unset($commits[$key]);
  continue;
}

...to drop the result.