How to merge all files manually in Git?

Question

I want to merge all files manually with meld or any other diff tool, how can I do this with Git?
When I run git mergetool it says no

Answer 1

I had a scenario where:

git merge --no-commit merge_branch 

just caused a Fast Forward.

If this happens you can use:

git merge --no-commit --no-ff merge_branch

and then you'll be able to review your changes