In Maniphest tasks, only move old owner to CC if owner changed

Summary:
Fixes T10426. When the owner of a task changes, we try to add the old owner to CC so they're kept in the loop.

Currently, we do this unconditionally. This can add the owner as a subscriber when someone didn't change anything, which is confusing.

Instead, only do this if the owner actually changed.

Test Plan:

• With "A" as owner, edited task and saved.
  • Before patch, A was added as subscriber.
  • After patch, A not added.
• With "A" as owner, changed owner to "B" and saved.
  • Both before and after patch, "A" is added as a subscriber.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10426

Differential Revision: https://secure.phabricator.com/D15333