There is no such functionality directly in PhpStorm, but you may try the following workaround:
git checkout master
git merge --no-commit <branch>
- now you have all the difference between your branch and master as uncommitted local changes.
- Invoke "Inspect Code"
- Select "Uncommitted Changes: All"
git commit
# commit the merge if needed (or just reset all if you don't need to merge now)