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 isn’t any divergent work to merge together—this is called a “fast-forward.”
For more: 3.2 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.