xavierchia

# Tell system when Xcode utilities live:
sudo xcode-select -switch /Applications/Xcode.app/Contents/Developer

# Set "opendiff" as the default mergetool globally:
git config --global merge.tool opendiff

# Then, when git tells you that there is a conflict, you can run `git mergetool` and 
# it will open XCode's mergetool (like in the question's screenshot) and you can work through the conflict.