Optimize the algorithm used to create the diff used to open a new PR
    
    Here as well, we used to iterate through all the commits to find which
    ones are present on the other branch. Now we just iterate through both
    branch and stop when we encounter the first commit present in both
    branches.
    
    We will need to consolidate our code base a little as this is the first
    time we are using a similar piece of code, so we definitively have
    duplication here.