keg: fix fallback dependencies with multiple kegs

Fixes #1524.

Library/Homebrew/keg.rb

@@ -105,7 +105,13 @@ class Keg
     #
     # This happens after the initial dependency check because it's sloooow.
     remaining_formulae = Formula.installed.select do |f|
-      f.installed_kegs.any? { |k| Tab.for_keg(k).runtime_dependencies.nil? }
+      installed_kegs = f.installed_kegs
+
+      # All installed kegs are going to be removed anyway,
+      # so it doesn't matter what they depend on.
+      next false if (installed_kegs - kegs).empty?
+
+      installed_kegs.any? { |k| Tab.for_keg(k).runtime_dependencies.nil? }
     end
 
     keg_names = kegs.map(&:name)

Library/Homebrew/test/keg_test.rb

@@ -382,6 +382,13 @@ class InstalledDependantsTests < LinkTestCase
     assert_equal [[@keg], ["bar"]], Keg.find_some_installed_dependents([@keg])
   end
 
+  def test_uninstalling_dependent_and_dependency
+    dependencies nil
+    Formula["bar"].class.depends_on "foo"
+    assert_empty @keg.installed_dependents
+    assert_nil Keg.find_some_installed_dependents([@keg, @dependent])
+  end
+
   def test_empty_dependencies_in_tab
     dependencies []
     assert_empty @keg.installed_dependents