Revert "Changes the query string parameter `sha` to `ref` to reflect the GitLab API"

Fixes #6344, refs #6198

This reverts commit 8a496cd2a7.

src/Composer/Repository/Vcs/GitLabDriver.php

@@ -175,7 +175,7 @@ class GitLabDriver extends VcsDriver
      */
     public function getDist($identifier)
     {
-        $url = $this->getApiUrl().'/repository/archive.zip?ref='.$identifier;
+        $url = $this->getApiUrl().'/repository/archive.zip?sha='.$identifier;
 
         return array('type' => 'zip', 'url' => $url, 'reference' => $identifier, 'shasum' => '');
     }

tests/Composer/Test/Repository/Vcs/GitLabDriverTest.php

@@ -144,7 +144,7 @@ JSON;
         $reference = 'c3ebdbf9cceddb82cd2089aaef8c7b992e536363';
         $expected = array(
             'type' => 'zip',
-            'url' => 'https://gitlab.com/api/v3/projects/mygroup%2Fmyproject/repository/archive.zip?ref='.$reference,
+            'url' => 'https://gitlab.com/api/v3/projects/mygroup%2Fmyproject/repository/archive.zip?sha='.$reference,
             'reference' => $reference,
             'shasum' => '',
         );