Merge pull request #21603 from Eric-Guo/dev_edge_common

dev and edge have some common, so factor it out.

railties/lib/rails/generators/app_base.rb

@@ -205,24 +205,21 @@ def self.path(name, path, comment = nil)
       end
 
       def rails_gemfile_entry
-        if options.dev?
-          [
-            GemfileEntry.path('rails', Rails::Generators::RAILS_DEV_PATH),
+        dev_edge_common = [
             GemfileEntry.github('sprockets-rails', 'rails/sprockets-rails'),
             GemfileEntry.github('sprockets', 'rails/sprockets'),
             GemfileEntry.github('sass-rails', 'rails/sass-rails'),
             GemfileEntry.github('arel', 'rails/arel'),
             GemfileEntry.github('rack', 'rack/rack')
           ]
+        if options.dev?
+          [
+            GemfileEntry.path('rails', Rails::Generators::RAILS_DEV_PATH)
+          ] + dev_edge_common
         elsif options.edge?
           [
-            GemfileEntry.github('rails', 'rails/rails'),
-            GemfileEntry.github('sprockets-rails', 'rails/sprockets-rails'),
-            GemfileEntry.github('sprockets', 'rails/sprockets'),
-            GemfileEntry.github('sass-rails', 'rails/sass-rails'),
-            GemfileEntry.github('arel', 'rails/arel'),
-            GemfileEntry.github('rack', 'rack/rack')
-          ]
+            GemfileEntry.github('rails', 'rails/rails')
+          ] + dev_edge_common
         else
           [GemfileEntry.version('rails',
                             Rails::VERSION::STRING,