We follow a different merge procedure for each attachment type:
- For PDF attachments, compare by MD5. If no match, get the top 50 words
in the attachment's text and hash those, then check again for a match.
Update references to item keys in notes and annotations.
- For web (snapshot / link) attachments, compare by title and URL.
Prefer a title + URL match but accept a title-only match.
- For other attachment types, keep all attachments from all items being
merged.
Also:
- Move most merge tests from Duplicates to Items#merge(). It just doesn't
make sense to worry about the UI in these.
