Update regex to use union

lib/gitlab/file_detector.rb

@@ -8,7 +8,7 @@ module Gitlab
   module FileDetector
     PATTERNS = {
       # Project files
-      readme: %r{\A(readme|index)(|\.#{Gitlab::MarkupHelper::EXTENSIONS.join('|\.')})$[^\/]*\z}i,
+      readme: %r{\A(#{Regexp.union(*Gitlab::MarkupHelper::PLAIN_FILENAMES).source})(\.(#{Regexp.union(*Gitlab::MarkupHelper::EXTENSIONS).source}))?$[^\/]*\z}i,
       changelog: %r{\A(changelog|history|changes|news)[^/]*\z}i,
       license: %r{\A((un)?licen[sc]e|copying)(\.[^/]+)?\z}i,
       contributing: %r{\Acontributing[^/]*\z}i,