Update `differential.close` to use DifferentialTransactionEditor

Summary: Ref T2222. Straightforward update to new stuff.

Test Plan:
  - Tried to close an uncloseable revision.
  - Closed a closeable revision.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8425