Use git cache path to mirror the repository before download to vendor target.

src/Composer/Downloader/GitDownloader.php

@@ -43,10 +43,21 @@ 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) ? '--dissociate --reference '.ProcessExecutor::escape($cachePath).' ' : '';
-
         $ref = $package->getSourceReference();
         $flag = Platform::isWindows() ? '/D ' : '';
+
+        // --dissociate option is only available since git 2.3.0-rc0
+        if (version_compare($this->gitUtil->getVersion(), '2.3.0-rc0', '>=')) {
+            if (!file_exists($cachePath)) {
+                $this->io->writeError(sprintf('    Cloning to cache at %s', ProcessExecutor::escape($cachePath)));
+                $mirrorCommand = 'git clone --mirror %s %s';
+                $mirrorCommandCallable = function ($url) use ($cachePath, $mirrorCommand) {
+                    return sprintf($mirrorCommand, ProcessExecutor::escape($url), ProcessExecutor::escape($cachePath));
+                };
+                $this->gitUtil->runCommand($mirrorCommandCallable, $url, $path, true);
+            }
+        }
+        $cacheOptions = file_exists($cachePath) ? '--dissociate --reference '.ProcessExecutor::escape($cachePath).' ' : '';
         $command = 'git clone --no-checkout %s %s '.$cacheOptions.'&& cd '.$flag.'%2$s && git remote add composer %1$s && git fetch composer';
         $this->io->writeError("    Cloning ".$ref);