GitDownloader: combine checkout + reset commands into a single process

use a single process instead of 3 to improve performance

src/Composer/Downloader/GitDownloader.php

@@ -454,15 +454,12 @@ class GitDownloader extends VcsDownloader implements DvcsDownloaderInterface
 
             $command = sprintf('git checkout %s --', ProcessExecutor::escape($branch));
             $fallbackCommand = sprintf('git checkout '.$force.'-B %s %s --', ProcessExecutor::escape($branch), ProcessExecutor::escape('composer/'.$branch));
-            if (0 === $this->process->execute($command, $output, $path)
+            $resetCommand = sprintf('git reset --hard %s --', ProcessExecutor::escape($reference));
-                || 0 === $this->process->execute($fallbackCommand, $output, $path)
+
-            ) {
+            if (0 === $this->process->execute("($command || $fallbackCommand) && $resetCommand", $output, $path)) {
-                $command = sprintf('git reset --hard %s --', ProcessExecutor::escape($reference));
-                if (0 === $this->process->execute($command, $output, $path)) {
                 return null;
             }
         }
-        }
 
         $command = sprintf($template, ProcessExecutor::escape($gitRef));
         if (0 === $this->process->execute($command, $output, $path)) {