Fixes #3388 by moving re-entrant `shared_status_from_xml` before transaction block (#3622)

Steps to reproduce the original issue:
1. Have two remote accounts, A that you don't follow, and B that you follow.
2. Have A post a toot and reply to it.
3. Boost A's reply from remote account B.

This used to cause the local instance to get A's reply but fail to link it to
the original post.