没有错……
如果你使用git help merge，就会看到git merge的文档。
然后它提到：

Then "git merge topic" will replay the changes made on the topic branch since it perged from master (i.e., E) until its current commit (C) on top of master,and record the result in a new commit along with the names of the two parent commits and a log message from the user describing the changes.

merge前

                  A---B---C topic
                /
           D---E---F---G master

merge后

                 A---B---C topic
                /         \
           D---E---F---G---H master

所以说，那个commit message是自动添加的。

如果你实在讨厌这个commit，可以在merge时添加--no-commit。这样就不会产生commit message了。不过我不推荐这么做，这样的话，就不容易区分merge的结果了。