qdiff: migrate to modern pager API This results in the default pager-attend list being empty. Sadly, we can't let the code be that way, because some legacy extensions depend on hooking the pager's attend list at import time (and we'd like to not break them), and if the list is actually *empty* that triggers magic behavior in the extension that attends everything. Instead, we put a long, improbable command name as the only entry in the attend list.

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