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

Note, if you insists of merging manually (perhaps for a certain class of files), you still can define a merge driver.
You have a concrete example in "Git - how to force merge conflict and manual merge on selected file".

That way, your merge driver script can call any merge tool you want.