Allow use of Github OAuth2 token stored in git config

src/Composer/Repository/Vcs/GitHubDriver.php

@@ -350,6 +350,15 @@ class GitHubDriver extends VcsDriver
 
     protected function authorizeOAuth()
     {
+		// If available use token from git config
+		exec('git config github.accesstoken', $output, $returnCode);
+		if ($returnCode === 0 && !empty($output[0]))
+		{
+			$this->io->write('Using Github OAuth token stored in git config (github.accesstoken)');
+			$this->io->setAuthorization($this->originUrl, $output[0], 'x-oauth-basic');
+			return;
+		}
+
         $attemptCounter = 0;
 
         $this->io->write('The credentials will be swapped for an OAuth token stored in '.$this->config->get('home').'/config.json, your password will not be stored');