audit: use .diff instead of .patch for github diffs

Library/Homebrew/cmd/audit.rb

@@ -311,6 +311,8 @@ class FormulaAuditor
       end
     when %r[macports/trunk]
       problem "MacPorts patches should specify a revision instead of trunk:\n#{patch.url}"
+    when %r[^https?://github\.com/.*commit.*\.patch$]
+      problem "GitHub appends a git version to patches; use .diff instead."
     end
   end