Create a patch of your Branch with all Trunk changes included, you can move those changes to a new branch. And you can rebase as needed to keep a clean commit history.
git diff Trunk Branch1 > ../patchfile
git checkout Branch2
git apply ../patchfile
If you don't care about your commit history, you can soft reset to convert it to a single commit. Example combines 10 commits.
git reset --soft HEAD~10
git commit -m "Combined feature changes."
git push origin my-branch --force