Keg#unlink: check file comes from the right keg

For real this time.

This makes sure that files won't be unlinked if they resolve into 
other kegs. For example: if fish and fishfish are both installed, 
with fishfish linked and fish unlinked, `brew unlink fish` would 
unlink some files that resolve into fishfish's keg.

Library/Homebrew/keg.rb

@@ -34,7 +34,12 @@ class Keg < Pathname
       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 != dst.resolved_path
+          next
+        end
+
         dst.uninstall_info if dst.to_s =~ INFOFILE_RX and ENV['HOMEBREW_KEEP_INFO']
         dst.unlink
         dst.parent.rmdir_if_possible
@@ -79,6 +84,8 @@ class Keg < Pathname
   def link mode=nil
     raise "Cannot link #{fname}\nAnother version is already linked: #{linked_keg_record.realpath}" if linked_keg_record.directory?
 
+    # these are used by the ObserverPathnameExtension to count the number
+    # of files and directories linked
     $n=0
     $d=0