meld

~/.gitconfig

[merge]
	tool = meld
[mergetool "meld"]
	cmd = meld "$LOCAL" "$MERGED" "$REMOTE" --output "$MERGED"

# Uncomment the following if you also want to be able to use meld on regular diffs with `git difftool` where you would normally use `git diff`
# [diff]
# 	tool = meld
# [difftool]
# 	prompt = false
# [difftool "meld"]
# 	cmd = meld "$LOCAL" "$REMOTE"

brew cask install meld # to install

git mergetool # when git tells you there's a merge conflict