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/Views/Source.php

@@ -434,8 +434,7 @@ class IDF_Views_Source
         $commit = trim($match[2]);
         $scm = IDF_Scm::get($request->project);
         $base = $request->project->shortname.'-'.$commit;
-        $cmd = $scm->getArchiveCommand($commit, $base.'/');
-        $rep = new Pluf_HTTP_Response_CommandPassThru($cmd, 'application/x-zip');
+        $rep = $scm->getArchiveStream($commit, $base.'/');
         $rep->headers['Content-Transfer-Encoding'] = 'binary';
         $rep->headers['Content-Disposition'] = 'attachment; filename="'.$base.'.zip"';
         return $rep;