In a local folder with several files, I have a git repository for which branch x only includes a few of those files, and the master one inc
git checkout -f branch_you_want_to_go
CAREFUL. This will revert ALL changes done on the branch. But if you are forced to merge and doesnt let you stash is an easy way when not much is done.
Do this if you want to delete your branch afterwards (probably with git branch -D branch_to_delete) or you want to start working on this brand from 0.