Pholio - fix a bug replacing multiple images

Summary: these transactions should //never// merge since they are always created for a 1:1 replacement. (ie any merging would be implicitly erroneous).  Fixes T4081

Test Plan: made a mock with three images and replaced all three successfully. replaced image A with image B, did not save, replaced image B with image C, then saved and verified transaction correctly showed image A replaced with image C.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4081

Differential Revision: https://secure.phabricator.com/D7536