Under what exact condition does Git perform a fast-forward merge, rather than creating a new merge commit?

Git performs a fast-forward merge when the tip commit of the branch being merged into (the destination branch) is a direct ancestor of the tip commit of the branch being merged (the source branch). This specific condition means that the destination branch has not diverged from the source branch; all commits on the destination branch are already present in the history of the source branch, and no new....