formula: make cache clearing methods, *ahem*, clearer

Library/Homebrew/formula.rb

@@ -1337,10 +1337,14 @@ class Formula
     end
   end
 
-  # Clear caches of .racks and .installed.
-  # @private
-  def self.clear_cache
+  # Clear cache of .racks
+  def self.clear_racks_cache
     @racks = nil
+  end
+
+  # Clear caches of .racks and .installed.
+  def self.clear_installed_formulae_cache
+    clear_racks_cache
     @installed = nil
   end
 

Library/Homebrew/tab.rb

@@ -319,7 +319,7 @@ class Tab < OpenStruct
   def write
     # If this is a new installation, the cache of installed formulae
     # will no longer be valid.
-    Formula.clear_cache unless tabfile.exist?
+    Formula.clear_installed_formulae_cache unless tabfile.exist?
 
     CACHE[tabfile] = self
     tabfile.atomic_write(to_json)