A rebase rewrites history, as if the PR had been written on master directly. There is no merge commit and no evidence of a previous branch. Some people prefer rebasing over merging because it gives you a linear commit history.
`git-rebase` will rewrite the history of the current branch. In this case, github will rewrite the history of the branch that you would like to merge such that the base commit (hence the name "rebase") or the commit that was branched off of is the latest commit on the default branch (usually master)
Then when you merge it, you'll get a merge commit or not depending on whether or not you do a fast-forward merge