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