Fixed issue #2595: GitLab OAuth2 provider URL misses api/v4/ suffix.

db/migrate/20190903165443_issue_2595_gitlab_placeholder.rb

@@ -0,0 +1,10 @@
+class Issue2595GitlabPlaceholder < ActiveRecord::Migration[5.2]
+  def change
+    # return if it's a new setup
+    return if !Setting.find_by(name: 'system_init_done')
+
+    setting = Setting.find_by(name: 'auth_gitlab_credentials')
+    setting.options['form'].last['placeholder'] = 'https://gitlab.YOURDOMAIN.com/api/v4/'
+    setting.save!
+  end
+end

db/seeds/settings.rb

@@ -1450,7 +1450,7 @@ Setting.create_if_not_exists(
         null:        true,
         name:        'site',
         tag:         'input',
-        placeholder: 'https://gitlab.YOURDOMAIN.com',
+        placeholder: 'https://gitlab.YOURDOMAIN.com/api/v4/',
       },
     ],
   },