cmd/install: check before tapping

Changes introduced in dfdc570abcd7c19a7b385c2fe8c6b26b12bca646 were
causing installations using fully-qualified formula names to fail.

Library/Homebrew/cmd/install.rb

@@ -19,7 +19,7 @@ module Homebrew
         user = $1
         repo = $2.sub(/^homebrew-/, "")
         tap = Tap.fetch(user, repo)
-        tap.install
+        tap.install unless tap.installed?
       end
     end unless ARGV.force?