Keg#unlink: check destination before unlinking

Rather than skip unlinking if there's no linked keg record, check to
see whether the destination's realpath is the same as the source file
in the keg being unlinked.

Library/Homebrew/cmd/unlink.rb

@@ -3,7 +3,6 @@ module Homebrew extend self
     raise KegUnspecifiedError if ARGV.named.empty?
 
     ARGV.kegs.each do |keg|
-      return if !keg.linked?
       print "Unlinking #{keg}... "
       puts "#{keg.unlink} links removed"
     end

Library/Homebrew/keg.rb

@@ -29,14 +29,18 @@ class Keg < Pathname
 
   def unlink
     n=0
-    return n if !linked?
 
     %w[bin etc lib include sbin share var].map{ |d| self/d }.each do |src|
       next unless src.exist?
       src.find do |src|
         next if src == self
         dst=HOMEBREW_PREFIX+src.relative_path_from(self)
-        next unless dst.symlink?
+
+        # check whether the file to be unlinked is from the current keg first
+        if !dst.symlink? || !dst.exist? || src.expand_path != dst.realpath
+          next
+        end
+
         dst.uninstall_info if dst.to_s =~ INFOFILE_RX and ENV['HOMEBREW_KEEP_INFO']
         dst.unlink
         dst.parent.rmdir_if_possible