Cannot update upstream merged conflict. Error from git pull -Xtheirs git://github.com/pantheon-systems/WordPress.git master:

gistfile1.txt

"Let’s try these steps from your local environment.

1.) Backup the wp-content folder (save it somewhere). Then delete folder that folder.
2.) Reset the site code base (undo commit) to the last applied core updates / successfully update (you can find the commit hash / find something update to Wordpress in the site dashboard -> dev site.  -> code). If doesn’t work, find the next successful core updates. See. This guide on how undo commits: https://pantheon.io/docs/undo-commits/ <— 
Example:
git reset --hard d2043a0e
d2043a0e is the commit where I was able to update WordPress core in the past.
3.) git pull -Xtheirs git://github.com/pantheon-systems/WordPress.git master
4.) Restore the wp-content folder
5.) git push origin master
6.) If it doesn’t work, force it like git push -f origin master

If that doesn’t try these second batch of steps

*Procedure to undo-commits until to see working upstream updates*
Warning: Doing this will remove your changes, so make sure to make a back-up of your files that does not conflict with core files (/wp-content/* or /sites/all, /modules, /themes)

1. Clone the repository via Git locally. Optional: create a separate branch to test first
2. Run `git log --pretty=format:""%h - %an, %ar : %s""`
3. Look for the earliest commit that can tell upstream updates are still working and get the hash key of the commit
4. Run `git reset --hard <hash-key>` to reach to reset the repo to that commit
5. Push back to the environment as forced: `git push origin <branch> -f`
6. Run upstream updates to the environment and see if it will work
7.  In the local Git repo copy, run `git pull origin <branch>` to sync locally the upstream updates that worked
8. Add your custom code back to the repo and commit back."