When I open a pull request on GitHub.
All commits since my last request and all new ones are automatically added to this request
You can create Pull Request(PR), by making separate branches for your work.
Example:
You checkout from a branch master into a branch work-1.
You make some commits in branch work-1 as work-1-commit-1 and work-1-commit-2
Now you create a PR from work-1 to master. Your code can be reviewed by seeing files changed from PR.
Now, for further work you will checkout from branch work-1 into new branch work-2
You make some commits in branch work-2 as work-2-commit-1 and work-2-commit-2
Now you create a PR from work-2 to work-1. Your code can be reviewed by seeing files changed from PR.
Here the files changes will only have the new code you write after work-1-commit-2.