The commit in rEUME402404d4cedaedb9798122f9d5c779adc5fea808 causes an error reported in Topic:Todlzx0oqxc758g0 when merging watchlist entries (error happens if there are watchisted pages on both users being merged). See post-review message in https://gerrit.wikimedia.org/r/#/q/I18e898c4a55e0f5e553f94fa41967f8f54d07096
Once fixed needs backport to REL1_28
Steps to reproduce:
First you need 2 users, second ensure that their watchlist has the same page, then do merge and database error occurred.
The error also still exist in master branch.