extract: Log formula version

When extracting based on --git-revision the version of the formula is
not known to the user upfront.

Library/Homebrew/dev-cmd/extract.rb

@@ -147,7 +147,7 @@ module Homebrew
           odebug "Overwriting existing formula at #{path}"
           path.delete
         end
-        ohai "Writing formula for #{name} from revision #{rev} to:", path
+        ohai "Writing formula for #{name} at #{version} from revision #{rev} to:", path
         path.dirname.mkpath
         path.write result
       end