When landing changes in an empty repository, merge cleanly in Git

Summary:
Fixes T12876. Ref T13546. When you make the first change in a new Git repository, "arc land" currently can not merge it because there's nothing to merge into.

Support merging into the empty state formally, reachable by using "--into-empty" (which should be uncommon) or "arc land" in an empty repository.

Test Plan:

• 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.

Maniphest Tasks: T13546, T12876

Differential Revision: https://secure.phabricator.com/D21324