We have a job that automatically cuts the train branch each week.

The script that starts this process is: https://gitlab.wikimedia.org/repos/releng/release/-/blob/main/make-release/automatic-branch-cut

What it does:

• For all branched extensions, skins, and core it makes a new branch named wmf/<version>
• It checks out mediawiki/core
  • git branch -B wmf/<version>
  • Removes existing submodules (git submodule deinit && git rm)
  • Modifies extensions/.gitignore to remove the * line
  • Does git submodule add --branch <wmf/<version> for all branched extensions/skins
  • Removes vendor from .gitignore
  • Updates the version in includes/Defines.php
  • Pushes this as a new change to the branch (example)

There's a problem here.

If the HEAD of mediawiki/core changes between when the branch is created and when it's cloned, this process will fail with Implicit merge (see T373425)