Some images have huge amounts of XMP tag Xmp.photoshop.DocumentAncestors.
In certain cases more than 100000 values. This is apparently due to a bug
in certain versions of PhotoShop.
This makes deserializing it in the way we currently do too slow, probably
because of continuous reallocations after adding each value. Until we can
change this let's remove everything but the first 1000 values when
serializing. I think it is very unlikely there are any real cases where
more than a 1000 ancestor documents are referenced in an image. Testing
shows that this amount doesn't cause any serious slowdowns.
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