Rework the way IDF's SCM interface provides downloadable snapshots.

Instead of returning a command which gets executed and which should
pass through / stream its output data to the client, we're just
returning an instance of Pluf_HTTP_Response. This is needed, because
some SCMs, most noticable monotone, have no locally executable command
to provide a snapshot archive (and probably never will for our kind
of setup).

We therefor added a little BSD-licensed class "ZipArchive" which allows
the creation of pkzip-compatible archives on the fly by letting it eat
the file contents directly feed from the (remote) stdio instance.
Download performance is ok and lies between 15K/s and 110K/s, but at
least we do no longer block the browser while we pre-generate the zip
file server-side.

Thanks to Patrick Georgi for all his work!

src/IDF/Scm.php

@@ -393,13 +393,13 @@ class IDF_Scm
     }
 
     /**
-     * Generate the command to create a zip archive at a given commit.
+     * Generate a zip archive at a given commit, wrapped in a HTTP response, suitable for pushing to client.
      *
      * @param string Commit
      * @param string Prefix ('repository/')
-     * @return string Command
+     * @return Pluf_HTTP_Response The HTTP Response containing the zip archive
      */
-    public function getArchiveCommand($commit, $prefix='repository/')
+    public function getArchiveStream($commit, $prefix='repository/')
     {
         throw new Pluf_Exception_NotImplemented();
     }