That button only appears when you add new commits to the existing branch/PR. If you amend any commits and force push, the rewritten commits are lost forever on the GitHub side of things. You can only find them in your local ref-log at that point.

Gerrit instead retains each rewrite of the "same" commit. It does this by requiring you to insert a "Change-Id: ..." footer into each commit message (it provides a repo hook to do this) and examines each incoming commit message to know whether that commit is a revision to a commit it's already seen.