Build relative paths using resolved_path and relative_path_from

Library/Homebrew/cmd/update.rb

@@ -17,12 +17,12 @@ module Homebrew extend self
     cd HOMEBREW_REPOSITORY
     git_init_if_necessary
 
-    tapped_formulae = Dir['Library/Formula/*'].map do |formula|
-      path = Pathname.new formula
+    taps = HOMEBREW_LIBRARY.join("Taps")
+    tapped_formulae = []
+    HOMEBREW_LIBRARY.join("Formula").children.each do |path|
       next unless path.symlink?
-      Pathname.new(path.realpath.to_s.gsub(/.*Taps\//, '')) rescue nil
+      tapped_formulae << path.resolved_path.relative_path_from(taps)
     end
-    tapped_formulae.compact!
     unlink_tap_formula(tapped_formulae)
 
     report = Report.new