When diffing the branches, do not browse master all the time, once is enough
    
    We used to iterate through the branches, then browse master/HEAD then
    browse the branch to see if it has commits that aren't in master/HEAD.
    With this commit, we'll browse master/HEAD just once at the start, then
    iterate through the branches, skip the branch that is equivalent to
    master/HEAD and check the other to see if they have commits not present
    in master.