Feature summary (what you would like to be able to do and where):
Allow 3-way merge from arbitrary revisions. The function removed in T297322 https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/core/+/7b0c94fb0be44be33393e34cf8bc73798105f78e%5E%21/includes/EditPage.php
(Copied from T58184)
This allows the following cool "editprotected" flow:
- Requester puts the modified version of the requested page somewhere else (maybe a subpage of talk page).
- Requester tells admins revision ids of the modified version and the original revision which the modified one is based on, possibly with a gadget.
- An admin edits the page with &undoafter=<modified revid>&undo=<original revid>.
- If there's a conflict, admins can check &diff=<modified revid>&oldid=<original revid> and manually apply that, or ask the requester to submit a new "patch".