It is handy to squash down your commits before merging your PR with my-new-cool-feature. You can either squash them down by doing an interactive rebase like so:

$ git checkout my-new-cool-feature
$ git rebase -i master

This will open up your $EDITOR of choice and you are free to pick and choose which commits to squash together.

This might be tedious if you have a big number of commits to squash together. Very tedious. TIL that you can use git merge to squash your commits, all in one go.

$ git checkout master
$ git merge --squash my-cool-new-feature

This will leave all of your changes staged on master, ready to be committed as one.