with many Xmp.photoshop.DocumentAncestors tags
This is similar to #7464, but in this case the XMP metadata was already
included in an XCF image.
We check for the occurrence of Xmp.photoshop.DocumentAncestors and stop
handling values when there are more than a 1000.
It would be easier to just check length for all tags and always
ignore when there are more than a 1000 values.
But in that case we would need to be sure there are no valid reasons for
tags to occur more than a 1000 times. So let's just limit it to this
specific tag.
(cherry picked from commit cadf485299)
# Conflicts:
# libgimpbase/gimpmetadata.c
8c1b71120e