keg: fix dependents with unavailable formulae

Because of an accidental use of `=` instead of `==`, the source formula
check would be skipped when determining if a keg depended on another one
(so only the versions would be compared).

Fixed that comparison, and updated the corresponding test.

Glad I caught that!

Library/Homebrew/keg.rb

@@ -357,7 +357,7 @@ class Keg
   end
 
   def installed_dependents
-    my_tab = Tab.for_keg(self)
+    tap = Tab.for_keg(self).source["tap"]
     Keg.all.select do |keg|
       tab = Tab.for_keg(keg)
       next if tab.runtime_dependencies.nil? # no dependency information saved.
@@ -368,7 +368,7 @@ class Keg
           dep_formula = Formulary.factory(dep["full_name"])
           next false unless dep_formula == to_formula
         rescue FormulaUnavailableError
-          next false unless my_tab["full_name"] = dep["full_name"]
+          next false unless "#{tap}/#{name}" == dep["full_name"]
         end
 
         dep["version"] == version.to_s

Library/Homebrew/test/keg_test.rb

@@ -360,9 +360,11 @@ class InstalledDependantsTests < LinkTestCase
   # from a file path or URL.
   def test_unknown_formula
     Formulary.unstub(:loader_for)
-    alter_tab { |t| t.source["tap"] = "some/tap" }
-    dependencies [{ "full_name" => "some/tap/bar", "version" => "1.0" }]
-    alter_tab { |t| t.source["path"] = nil }
+    alter_tab(@keg) do |t|
+      t.source["tap"] = "some/tap"
+      t.source["path"] = nil
+    end
+    dependencies [{ "full_name" => "some/tap/foo", "version" => "1.0" }]
     assert_equal [@dependent], @keg.installed_dependents
     assert_equal [[@keg], ["bar 1.0"]], Keg.find_some_installed_dependents([@keg])
   end