Don't return nil from Formula#linked_keg

Doing so was the result of a quick hack to fix the "deps installed as
upgrades don't get linked" bug, but it was a mistake. Instead, always
return the LinkedKegs entry as a Pathname object, and let callers be
responsible for checking that it exists.

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

Library/Homebrew/cmd/install.rb

@@ -12,7 +12,7 @@ module Homebrew extend self
     end unless ARGV.force?
 
     ARGV.formulae.each do |f|
-      if File.directory? HOMEBREW_REPOSITORY/"Library/LinkedKegs/#{f.name}"
+      if f.linked_keg.directory?
         raise "#{f} already installed\nTry: brew upgrade #{f}"
       end
     end unless ARGV.force?

Library/Homebrew/formula.rb

@@ -69,8 +69,7 @@ class Formula
   end
 
   def linked_keg
-    keg = Pathname.new(HOMEBREW_REPOSITORY/"Library/LinkedKegs"/@name)
-    if keg.exist? then Keg.new(keg.realpath) else nil end
+    HOMEBREW_REPOSITORY/'Library/LinkedKegs'/@name
   end
 
   def installed_prefix

Library/Homebrew/formula_installer.rb

@@ -66,7 +66,7 @@ class FormulaInstaller
     fi.show_header = false
     oh1 "Installing #{f} dependency: #{dep}"
     fi.install
-    dep.linked_keg.unlink if dep.linked_keg
+    Keg.new(dep.linked_keg.realpath).unlink if dep.linked_keg.directory?
     fi.caveats
     fi.finish
   end

Library/Homebrew/tab.rb

@@ -46,7 +46,7 @@ class Tab < OpenStruct
 
   def self.for_formula f
     f = Formula.factory f unless f.kind_of? Formula
-    path = HOMEBREW_REPOSITORY + 'Library' + 'LinkedKegs' + f.name + 'INSTALL_RECEIPT.json'
+    path = f.linked_keg/'INSTALL_RECEIPT.json'
 
     if path.exist?
       self.from_file path