Fix keep_files to be used as paths relative to the destination.

They were used as keywords to match files containing them in the paths.

lib/jekyll/cleaner.rb

@@ -94,11 +94,12 @@ module Jekyll
     # Private: Creates a regular expression from the config's keep_files array
     #
     # Examples
-    #   ['.git','.svn'] creates the following regex: /\/(\.git|\/.svn)/
+    #   ['.git','.svn'] with site.dest "/myblog/_site" creates 
+    #   the following regex: /\/myblog\/_site\/(\.git|\/.svn)/
     #
     # Returns the regular expression
     def keep_file_regex
-      Regexp.union(site.keep_files)
+      /#{Regexp.quote(site.dest)}\/(#{Regexp.union(site.keep_files).source})/
     end
   end
 end