git pull myRepo master. Git will complain.
git mergetool -- myFileName(assuming you already set up BC, per the link above)
View->LineNumbersto make things clearer.
git pull thatRepo master. If the changes are more significant, however, you will need to tell Git, line-by-line which code you want to keep. Here is the sequence of steps.
git mergetool -- yourfile.py
git commit -m "Fix merge conflict."