Resource diffs can only load the current copies of existing files

[LilithSilver]

While using the editor, resource diffs for an edited .png work just fine. I can add, edit, and delete files, and they all work as expected.

However, once the editor is restarted, the diffs mostly stop working. Occasionally, a current file will be shown, but rarely.

Printed to the log is:

 linked file "res://assets/something.png" not found

I've ensured that the arguments sent to get_files_on_branch_at from the diff system are always correct, so the issue is likely with that function and/or with the underlying data storage.

[LilithSilver]

Update: the issue is indeed with the underlying storage; we only keep a doc ID reference to the current copy of binary files, so can only ever retrieve the canonical version. #55 fixes this but does not scale well.