I guess there is no any button which shows you a complete list of commits. If you want to list all commits in a repo, you could browse the following URL:
https://github.com/username/repository/commits
You can view the list of commits by adding the word commits
(in plural) at the end of repo URL .
Optionally, you could add some query string to narrow the results in the list. For example:
https://github.com/username/repository/commits?author=johndoe
Update
Thanks to @lii I update this post:
If you want to view all commits in a branch, browse the following URL:
https://github.com/username/repository/commits/branch-name
And you could narrow the list of commits by browsing the following URL:
https://github.com/username/repository/commits/branch-name?author=johndoe