Allow for leading slash in path part of GitHub URLs

src/Composer/Repository/Vcs/GitHubDriver.php

@@ -47,7 +47,7 @@ class GitHubDriver extends VcsDriver
      */
     public function initialize()
     {
-        preg_match('#^(?:(?:https?|git)://([^/]+)/|git@([^:]+):)([^/]+)/(.+?)(?:\.git|/)?$#', $this->url, $match);
+        preg_match('#^(?:(?:https?|git)://([^/]+)/|git@([^:]+):/?)([^/]+)/(.+?)(?:\.git|/)?$#', $this->url, $match);
         $this->owner = $match[3];
         $this->repository = $match[4];
         $this->originUrl = !empty($match[1]) ? $match[1] : $match[2];
@@ -267,7 +267,7 @@ class GitHubDriver extends VcsDriver
      */
     public static function supports(IOInterface $io, Config $config, $url, $deep = false)
     {
-        if (!preg_match('#^((?:https?|git)://([^/]+)/|git@([^:]+):)([^/]+)/(.+?)(?:\.git|/)?$#', $url, $matches)) {
+        if (!preg_match('#^((?:https?|git)://([^/]+)/|git@([^:]+):/?)([^/]+)/(.+?)(?:\.git|/)?$#', $url, $matches)) {
             return false;
         }