add `git ff`

gitconfig

@@ -48,4 +48,5 @@
 
 [alias]
 	short = log --graph --pretty=format:'%C(yellow)%h%Creset -%C(red)%d%Creset %s %Cgreen(%cr) %C(bold blue)<%an>%Creset' --abbrev-commit --date=relative --patience
+	ff = merge --ff-only
 	put = !git-push && git-push --tags