git config --global merge.tool meld
git config --global diff.tool meld
or edit ~/.gitconfig
References
https://git-scm.com/book/en/v2/Customizing-Git-Git-Configuration
http://stackoverflow.com/questions/6412516/configuring-diff-tool-with-gitconfig
http://stackoverflow.com/questions/34119866/setting-up-and-using-meld-as-your-git-difftool-and-mergetool