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 insalled 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.

Closes #18495.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
vim 7.3.865