Fix mention processing for unknwon accounts on incoming ActivityPub N…

…otes (mastodon#10125)

`::FetchRemoteAccountService` is not `ActivityPub::FetchRemoteAccountService`,
its second argument is the pre-fetched body. Passing `id: false` actually passed
a `Hash` as the prefetched body, instead of properly resolving unknown remote
accounts.

app/lib/activitypub/activity/create.rb

@@ -159,7 +159,7 @@ def process_mention(tag)
     return if tag['href'].blank?
 
     account = account_from_uri(tag['href'])
-    account = ::FetchRemoteAccountService.new.call(tag['href'], id: false) if account.nil?
+    account = ::FetchRemoteAccountService.new.call(tag['href']) if account.nil?
 
     return if account.nil?