Editing a taxonomy term removes "related terms" references from other terms

If you edit Term 1, any other terms that reference Term 1 as a "related term" will lose that reference.

In other words, before you edit Term 1, the term_relation table might look like this:

tid1 tid2
1 2
2 1

But after you edit and save Term 1, the term_relation table will look like this:

tid1 tid2
1 2

The effects of this have been mostly hidden because if you edit Term 2, the reference to Term 1 will get recreated. (This is probably why the bug has persisted for so long.)

However, the bug is a real problem if you have a View or some other block that shows a term's related terms. Simply editing the description of Term 1 will suddenly cause the "related terms" block in Term 2 to be empty.

The attached patch modifies taxonomy.module to keep any back references to the original term. While it requires two more SQL queries, it keeps the term_relation table consistent.