Fix `brew update` so that it doesn't show doubles

Under certain conditions, `brew update` will show doubles. It's a rare
edge-case, but if you have a tap where the name of the formula is identical
with the name of the installed file, then you see output like this:

    ==> Updated Formulae
    aalib				           ssh-copy-id
    casperjs				       sshuttle
    jack				           telemachus/anytap/brew-any-tap
    mogenerator				       telemachus/anytap/brew-any-tap

I believe this should fix that.