R1:befeb17f6ff1

Improve the construction of synthetic "comparison/intradiff" changesets

Summary:
Ref T13523. Currently, when building a "comparison" changeset, metadata is taken from the left changeset. This is somewhat arbitrary.

This means that intradiffs of images don't work properly because the rendered changeset has only the left (usually "old") information.

Later, some of the code attempts to ignore the file data stored on the changeset and reconstruct the correct file data, which is how the result ends up not…