Fix incorrect behavior when deduplicating meta-Diff Files
As a result of a recent prod regression, some incorrect code was exposed as a result of testing an untested code path.
We have almost all of the pieces necessary to add a unique ID for all diff files:
- A consistent
commit
for each diff - A way to generate a unique ID for diff files (reverted)
To fix this incorrect test and prepare for a more reliable Diffs app, we should re-add the Unique ID and use it to correctly deduplicate files going through prepareDiffFiles
.