add Formula#pinned_version

Library/Homebrew/formula.rb

@@ -936,6 +936,11 @@ class Formula
     @pin.pinned?
   end
 
+  # @private
+  def pinned_version
+    @pin.pinned_version
+  end
+
   # @private
   def pin
     @pin.pin

Library/Homebrew/formula_pin.rb

@@ -33,4 +33,8 @@ class FormulaPin
   def pinnable?
     @f.rack.exist? && @f.rack.subdirs.length > 0
   end
+
+  def pinned_version
+    Keg.new(path.resolved_path).version if pinned?
+  end
 end