What is git fast
This question already has an answer here:
When you try to merge one commit with a commit that can be reached by following the first commit's history, Git simplifies things by moving the pointer forward because there is no divergent work to merge together – this is called a “fast-forward.”
For more : http://git-scm.com/book/en/v2/Git-Branching-Basic-Branching-and-Merging
In another way,
If Master has not diverged, instead of creating a new commit, git will just point master to the latest commit of the feature branch. This is a “fast forward.” There won't be any "merge commit" in fast-forwarding merge.
In Git, to "fast forward" means to update the HEAD pointer in such a way that its new value is a direct descendant of the prior value. In other words, the prior value is a parent, or grandparent, or grandgrandparent, ...
Fast forwarding is not possible when the new HEAD is in a diverged state relative to the stream you want to integrate. For instance, you are on master and have local commits, and git fetch has brought new upstream commits into origin/master . The branch now diverges from its upstream and cannot be fast forwarded: your master HEAD commit is not an ancestor of origin/master HEAD . To simply reset master to the value of origin/master would discard your local commits. The situation requires a rebase or merge.
If your local master has no changes, then it can be fast-forwarded: simply updated to point to the same commit as the latest origin/master . Usually, no special steps are needed to do fast-forwarding; it is done by merge or rebase in the situation when there are no local commits.
Is it ok to assume that fast-forward means all commits are replayed on the target branch and the HEAD is set to the last commit on that branch?
No, that is called rebasing, of which fast-forwarding is a special case when there are no commits to be replayed (and the target branch has new commits, and the history of the target branch has not been rewritten, so that all the commits on the target branch have the current one as their ancestor.)
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