Make updating a revision with the same active diff a no-op

Summary: Ref T13114. See PHI515. Updating a revision with the same, currently active diff became an error at some point (probably D19175). This is inconsistent; make it an allowable no-op instead.

Test Plan:

• Updated a revision's diff via Conduit.
• Updated to the same diff, no-op.
• Tried to update a different revision, error ("already attached elsewhere").
• Updated with a different diff.
• Tried to update with the original diff, error ("previously attached version").

Maniphest Tasks: T13114

Differential Revision: https://secure.phabricator.com/D19267