Rebase with fixup
The easiest way to clean up commit history is to use interactive rebase:
git rebase -i updated_main_branch
Then update the commit line by replacing the action with f for the commit you want to fold into the previous one.
Fixing ignored files that were already committed
If files should be ignored but are already tracked, first update .gitignore, then run:
git rm -r --cached . && git add . && git commit -am "Remove ignored files"