How combine git commits in one commit

Suppose you have many commits in branch feature/story_1 and need to prepare pull request with one commit to release/dev branch. For that use temporary branch to backup and take changes.

$ git checkout release/dev
$ git pull
$ git checkout feature/story_1
$ git checkout -b feature/story_1_backup
$ git checkout feature/story_1
$ git reset --hard release/dev
$ git merge --squash feature/story_1_backup

All changes would be in your files but NOT commited.