Automatically merge tags when uploading a duplicate.
There are two cases:
* You try to upload an image, but it's already on Danbooru. In this case
you'll be immediately redirected to the original post, before you
can start tagging the upload.
* You're uploading an image, it wasn't a dupe when you first opened the
upload page, but you got sniped while tagging it. In this case your tags
will be merged with the original post, and you will be redirected to the
original post.
There are a few corner cases:
* If you don't have permission to edit the original post, for example
because it's banned or has a censored tag, then your tags won't be
merged and will be silently ignored.
* Only the tags, rating, and parent ID will be merged. The source and
artist commentary won't be merged. This is so that if an artist uploads
the exact same file to multiple sites, the new source won't override
the original source.
* Some tags might be contradictory. For example, the new post might
be tagged translation_request, but the original post might already be
translated. It's up to the user to fix these things afterwards.
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