Serialize source/dest file information in validation_info
Review Request #10234 — Created Oct. 16, 2018 and discarded — Latest diff uploaded
validation_infopayloads now contain source and/or destination
information (depending on the operation, e.g., removals don't have
destinations since the file no longer exists).
This will allow us to compare the validation info to a cumulative diff.
Ran unit tests.