Fix possible array to string conversion renaming Pholio Mockup image

Summary:
Premising that the `$old` and `$new` variables are 1-element arrays defined as PHID=>title,
this can cause `renderValue()` repeatedly fail when passing an array instead of its value.

Thus pass `head($old)` instead, to get the first value - that is the only one, even if you rename
multiple images (since this Transaction is about a single Mockup image).

Closes T15646

Test Plan:
* Have `phd` running
* Create a Pholio mockup with at least one image

Edit the Pholio mockup and:

1. rename the Titles of an image
2. rename a single Image
3. rename no image

No nuclear implosions. You still see a lovely Feed mentioning each rename.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15646

Differential Revision: https://we.phorge.it/D25441

src/applications/pholio/xaction/PholioImageNameTransaction.php

@@ -31,8 +31,8 @@ final class PholioImageNameTransaction
       '%s renamed an image (%s) from %s to %s.',
       $this->renderAuthor(),
       $this->renderHandle(key($new)),
-      $this->renderValue($old),
+      $this->renderValue((string)head($old)),
-      $this->renderValue($new));
+      $this->renderValue((string)head($new)));
   }
 
   public function getTitleForFeed() {