When landing changes in an empty repository, merge cleanly in Git
Support merging into the empty state formally, reachable by using "--into-empty" (which should be uncommon) or "arc land" in an empty repository.
- Used "arc land --into-empty --hold ..." to generate merges against the empty state under "squash" and "merge" strategies in Git.
- Got sensible result commits with appropriate parents and content.
Differential Revision: https://secure.phabricator.com/D21324