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

A similar question is How to prevent an automerge using git?

FractalSpace gave an answer which I think useful:

$ git checkout master
$ git difftool -t kdiff3 local-branch HEAD

The idea is using difftools instead of auto-merging tools to manually pick what you need and create new files.