Feed Differential retitled: do not repeat the title twice

Summary:
This is a cosmetic modification for a specific Feed.

https://we.phorge.it/T15183

If you retitle a Diff (and you do nothing else), its Feed
is somehow too much verbose:

... retitled NEW_TITLE from OLD_TITLE to NEW_TITLE

In this specific case, the Feed now becomes shorter so that
the NEW_TITLE is not repeated twice:

... retitled NEW_TITLE from OLD_TITLE

Note that, if the title changes again, the Feed naturally comes
back as before so to mention the change and obviously without
any repetition. Example:

... retitled CURRENT_TITLE from OLD_TITLE to NEW_TITLE

Closes T15183

Test Plan:

• take a Diff named "OLD_TITLE" and retitle to "NEW_TITLE"
• visit the homepage and see "retitled NEW_TITLE from OLD_TITLE" (this means the new behavior works)
• retitle the Diff from "NEW_TITLE" to "CURRENT_TITLE"
• visit the homepage again and see "retitled CURRENT_TITLE from OLD_TITLE to NEW_TITLE" (this means the old behavior still works)

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: avivey, speck, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15183

Differential Revision: https://we.phorge.it/D25094