R1:f42dd5819e98

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…