Fast-forward and true merges

If nothing has happened on your branch since the other one split off, there is nothing to merge: git moves your branch label forward and stops. That is a fast-forward, and the history stays a straight line.

If both branches have moved, git builds a commit with two parents. The first parent is where you were, the second is what you brought in — and that order is why `git log --first-parent` can show you your own branch's story without the merged work interleaved.

`--no-ff` forces a merge commit even when a fast-forward was possible, which some teams use to keep a record that a branch existed at all.