keg: update installed dependency resolution

Closes #1525. (See there for a motivation for this change.)

Library/Homebrew/keg.rb

@@ -385,6 +385,7 @@ class Keg
   end
 
   def installed_dependents
+    return [] unless linked?
     tap = Tab.for_keg(self).source["tap"]
     Keg.all.select do |keg|
       tab = Tab.for_keg(keg)
@@ -394,12 +395,10 @@ class Keg
         # in case of conflicts between formulae from different taps.
         begin
           dep_formula = Formulary.factory(dep["full_name"])
-          next false unless dep_formula == to_formula
+          dep_formula == to_formula
         rescue FormulaUnavailableError
-          next false unless "#{tap}/#{name}" == dep["full_name"]
+          next "#{tap}/#{name}" == dep["full_name"]
         end
-
-        dep["version"] == version.to_s
       end
     end
   end

Library/Homebrew/test/keg_test.rb

@@ -340,6 +340,7 @@ class InstalledDependantsTests < LinkTestCase
   def setup
     super
     @dependent = setup_test_keg("bar", "1.0")
+    @keg.link
   end
 
   def alter_tab(keg = @dependent)
@@ -442,8 +443,8 @@ class InstalledDependantsTests < LinkTestCase
 
   def test_same_name_different_version_in_tab
     dependencies [{ "full_name" => "foo", "version" => "1.1" }]
-    assert_empty @keg.installed_dependents
-    assert_nil Keg.find_some_installed_dependents([@keg])
+    assert_equal [@dependent], @keg.installed_dependents
+    assert_equal [[@keg], ["bar 1.0"]], Keg.find_some_installed_dependents([@keg])
   end
 
   def test_different_name_same_version_in_tab
@@ -465,4 +466,11 @@ class InstalledDependantsTests < LinkTestCase
     assert_empty @keg.installed_dependents
     assert_equal [[@keg], ["bar"]], Keg.find_some_installed_dependents([@keg])
   end
+
+  def test_unlinked
+    @keg.unlink
+    dependencies [{ "full_name" => "foo", "version" => "1.0" }]
+    assert_empty @keg.installed_dependents
+    assert_nil Keg.find_some_installed_dependents([@keg])
+  end
 end