Try ssh before http auth, and make sure ssh fails fast if no login is possible

src/Composer/Downloader/GitDownloader.php

@@ -25,7 +25,7 @@ class GitDownloader extends VcsDownloader
     public function doDownload(PackageInterface $package, $path)
     {
         $ref = $package->getSourceReference();
-        $command = 'git clone %s %s && cd %2$s && git remote add composer %1$s && git fetch composer';
+        $command = 'git clone -c core.askpass=echo %s %s && cd %2$s && git remote add composer %1$s && git fetch composer';
         $this->io->write("    Cloning ".$ref);
 
         $commandCallable = function($url) use ($ref, $path, $command) {
@@ -140,7 +140,7 @@ class GitDownloader extends VcsDownloader
     {
         $handler = array($this, 'outputHandler');
 
-        // github, autoswitch protocols
+        // public github, autoswitch protocols
         if (preg_match('{^(?:https?|git)(://github.com/.*)}', $url, $match)) {
             $protocols = array('git', 'https', 'http');
             $messages = array();
@@ -162,7 +162,7 @@ class GitDownloader extends VcsDownloader
         $command = call_user_func($commandCallable, $url);
         if (0 !== $this->process->execute($command, $handler)) {
             if (preg_match('{^git@github.com:(.+?)\.git$}i', $url, $match) && $this->io->isInteractive()) {
-                // private repository without git access, try https with auth
+                // private github repository without git access, try https with auth
                 $retries = 3;
                 $retrying = false;
                 do {

src/Composer/Repository/Vcs/GitDriver.php

@@ -48,7 +48,7 @@ class GitDriver extends VcsDriver
                 $fs = new Filesystem();
                 $fs->removeDirectory($this->repoDir);
 
-                $command = sprintf('git clone --mirror %s %s', escapeshellarg($this->url), escapeshellarg($this->repoDir));
+                $command = sprintf('git clone -c core.askpass=echo --mirror %s %s', escapeshellarg($this->url), escapeshellarg($this->repoDir));
                 if (0 !== $this->process->execute($command, $output)) {
                     $output = $this->process->getErrorOutput();
 

src/Composer/Repository/Vcs/GitHubDriver.php

@@ -259,7 +259,8 @@ class GitHubDriver extends VcsDriver
                     case 401:
                     case 404:
                         $this->isPrivate = true;
-                        if (!$this->io->isInteractive()) {
+
+                        try {
                             // If this repository may be private (hard to say for sure,
                             // GitHub returns 404 for private repositories) and we
                             // cannot ask for authentication credentials (because we
@@ -274,6 +275,11 @@ class GitHubDriver extends VcsDriver
                             $this->gitDriver->initialize();
 
                             return;
+                        } catch (\RuntimeException $e) {
+                            if (!$this->io->isInteractive()) {
+                                $this->io->write('<error>Failed to clone the '.$this->generateSshUrl().' repository, try running in interactive mode so that you can enter your username and password</error>');
+                                throw $e;
+                            }
                         }
                         $this->io->write('Authentication required (<info>'.$this->url.'</info>):');
                         $username = $this->io->ask('Username: ');