All you have to do is:
git checkout feature_branch
git rebase master
git checkout master
git merge --squash feature_branch
As the docs for git merge --squash say:
Produce the working tree and index state as if a real merge happened (except for the merge information), but do not actually make a commit or move the HEAD, nor record $GIT_DIR/MERGE_HEAD to cause the next git commit command to create a merge commit. This allows you to create a single commit on top of the current branch whose effect is the same as merging another branch (or more in case of an octopus).
After that, you can git commit your changes which are already staged.