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.