brew.rb: don't restrict dev-cmd/*.rb to Homebrew developers.

Library/Homebrew/brew.rb

@@ -64,8 +64,13 @@ begin
   if cmd
     internal_cmd = require? HOMEBREW_LIBRARY_PATH.join("cmd", cmd)
 
-    if !internal_cmd && ARGV.homebrew_developer?
+    unless internal_cmd
       internal_cmd = require? HOMEBREW_LIBRARY_PATH.join("dev-cmd", cmd)
+      if internal_cmd && !ARGV.homebrew_developer?
+        safe_system "git", "config", "--file=#{HOMEBREW_REPOSITORY}/.git/config",
+                                     "--replace-all", "homebrew.devcmdrun", "true"
+        ENV["HOMEBREW_DEV_CMD_RUN"] = "1"
+      end
     end
   end