This simplest way is to switch to your local formtastic branch, then run git merge master
to merge the master branch changes in (you may have to deal with conflicts after that):
git branch formtastic
git merge master
If you want your history to be a little more structured, you can rebase instead:
git branch formtastic
git rebase -i master
Rebasing will make your history cleaner because the way it works is it takes whatever changes you made in formtastic and caches them, then in merges in new changes from master, then it re-plays your formtastic changes on top. This may take a little more work than simply merging though (and you'll have to look up rebasing to understand how it works).
Either way, once everything is conflict-free, tested, and committed in your branch, then you can go back and merge your changes into master like this:
git branch master
git merge formtastic