Use vcs dir as git reference, if existing, to speed up download

Also added home dir to default initial settings

src/Composer/Config.php

@@ -59,6 +59,7 @@ class Config
         'platform' => array(),
         'archive-format' => 'tar',
         'archive-dir' => '.',
+        'home' => '$HOME'
         // valid keys without defaults (auth config stuff):
         // bitbucket-oauth
         // github-oauth

src/Composer/Downloader/GitDownloader.php

@@ -42,10 +42,12 @@ class GitDownloader extends VcsDownloader implements DvcsDownloaderInterface
     {
         GitUtil::cleanEnv();
         $path = $this->normalizePath($path);
+        $cachePath = $this->config->get('cache-vcs-dir').'/'.preg_replace('{[^a-z0-9.]}i', '-', $url).'/';
+        $cacheOptions = file_exists($cachePath) ? $cacheOptions = '--reference '.$cachePath.' ' : '';
 
         $ref = $package->getSourceReference();
         $flag = Platform::isWindows() ? '/D ' : '';
-        $command = 'git clone --no-checkout %s %s && cd '.$flag.'%2$s && git remote add composer %1$s && git fetch composer';
+        $command = 'git clone --no-checkout %s %s '.$cacheOptions.'&& cd '.$flag.'%2$s && git remote add composer %1$s && git fetch composer';
         $this->io->writeError("    Cloning ".$ref);
 
         $commandCallable = function ($url) use ($ref, $path, $command) {