upgrade: unlink relative to the correct keg

Calling Keg#unlink on "#{f.rack}/#{f.version}" will perform the unlink
relative to the _new_ keg, rather than the keg we are upgrading from.
Fix this by resolving the linked_keg entry and unlinking relative to it.

Fixes Homebrew/homebrew#10296.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/cmd/upgrade.rb

@@ -39,7 +39,7 @@ module Homebrew extend self
       installer.show_header = false
       oh1 "Upgrading #{f.name}"
       installer.install
-      Keg.new("#{f.rack}/#{f.version}").unlink
+      Keg.new(f.linked_keg.realpath).unlink if f.linked_keg.directory?
       installer.caveats
       installer.finish # includes link step
     end