Fix a critical issue causing the Django Evolution entries to be wiped.
Fix a critical issue causing the Django Evolution entries to be wiped.

Our evolution initial data logic depended on FileDiff.parent_diff being
a valid database field. The recent addition of the diff
consolidation/hashing code renamed this, causing that to fail. The
consequence is that the evolution history got nuked.

Update the field so we don't totally lose everything. Anyone who ran
into this will need to recreate their DB from scratch. It should only
affect developers on master.

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