REVERT SubversionDownloadStrategy change.

Change 45ce0c963a52293063432674d5c360ebe65254e2 was done in error.
This submit reverts that change (which will go in a future commit.)

I apologize for the inconvenience.

Library/Homebrew/download_strategy.rb

@@ -119,47 +119,21 @@ end
 
 class SubversionDownloadStrategy <AbstractDownloadStrategy
   def fetch
-    # Looks like `svn up` is pretty cool, as it will save on bandwidth (makes 
-    # cache actually a cache) and will have a similar effect to verifying the 
-    # cache as it will make any changes to get the right revision.
     ohai "Checking out #{@url}"
-    if @spec == :revision
-      svncommand = @export.exist? ? 'up' : 'checkout';
-      args = [svn, svncommand, '--force', @url, @export]
-      args << '-r' << @ref if @ref
-      quiet_safe_system *args
-    elsif @spec == :revisions
-      externals = Hash.new
-      # Oh god escaping shell args.
-      # See http://notetoself.vrensk.com/2008/08/escaping-single-quotes-in-ruby-harder-than-expected/
-      `'#{svn.gsub(/\\|'/) { |c| "\\#{c}" }}' propget svn:externals \
-      '#{@url.gsub(/\\|'/) { |c| "\\#{c}" }}'`.each_line do |external_line|
-        key, value = external_line.split /\s+/
-        externals[key] = value
-      end
-      fetch_repo = lambda do |external, uri|
-        if external.to_s == @name
-          path = ''
+    @co=HOMEBREW_CACHE+@unique_token
+    unless @co.exist?
+      quiet_safe_system svn, 'checkout', @url, @co
     else
-          path = external.to_s
-        end
-        svncommand = (@export+path).exist? ? 'up' : 'checkout';
-        args = [svn, svncommand, '--force', '--ignore-externals', uri, @export+path]
-        args << '-r' << @ref[external] if @ref[external]
-        quiet_safe_system *args
-      end
-      fetch_repo.call @name, @url
-      externals.each_pair &fetch_repo
-    else
-      svncommand = @export.exist? ? 'up' : 'checkout';
-      args = [svn, svncommand, '--force', @url, @export]
-      quiet_safe_system *args
+      puts "Updating #{@co}"
+      quiet_safe_system svn, 'up', @co
     end
   end
 
   def stage
-    # `svn export PATH1 PATH2` doesn't need network when no revision is given.
-    quiet_safe_system svn, 'export', '--force', @export, Dir.pwd
+    # Force the export, since the target directory will already exist
+    args = [svn, 'export', '--force', @co, Dir.pwd]
+    args << '-r' << @ref if @spec == :revision and @ref
+    quiet_safe_system *args
   end
 
   # Override this method in a DownloadStrategy to force the use of a non-