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