formulary: recommend brew extract.

This will provide a better experience to users rather than a URL that
will likely break in future.

Library/Homebrew/formulary.rb

@@ -195,6 +195,15 @@ module Formulary
     end
 
     def load_file
+      if url =~ %r{githubusercontent.com/[\w-]+/[\w-]+/[a-f0-9]{40}(/Formula)?/([\w+-.@]+).rb}
+        formula_name = Regexp.last_match(2)
+        ohai "Consider using `brew extract #{formula_name} ...`!"
+        puts <<~EOS
+          This will extract your desired #{formula_name} version to a stable tap instead of
+          installing from an unstable URL!
+
+        EOS
+      end
       HOMEBREW_CACHE_FORMULA.mkpath
       FileUtils.rm_f(path)
       curl_download url, to: path