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/Monotone.php

@@ -21,8 +21,8 @@
 #
 # ***** END LICENSE BLOCK ***** */
 
-require_once(dirname(__FILE__) . "/Monotone/Stdio.php");
-require_once(dirname(__FILE__) . "/Monotone/BasicIO.php");
+//require_once(dirname(__FILE__) . "/Monotone/Stdio.php");
+//require_once(dirname(__FILE__) . "/Monotone/BasicIO.php");
 
 /**
  * Monotone scm class
@@ -135,6 +135,20 @@ class IDF_Scm_Monotone extends IDF_Scm
         return $branch;
     }
 
+    /**
+     * @see IDF_Scm::getArchiveStream
+     */
+    public function getArchiveStream($commit, $prefix='repository/')
+    {
+        $revs = $this->_resolveSelector($commit);
+        // sanity: this should actually not happen, because the 
+        // revision is validated before already
+        if (count($revs) == 0) {
+            return new Pluf_HTTP_Response_NotFound();
+        }
+        return new IDF_Scm_Monotone_ZipRender($this->stdio, $revs[0]); 
+    }
+
     /**
      * expands a selector or a partial revision id to zero, one or
      * multiple 40 byte revision ids