codereview.py: correct error handling without decorator

codereview.py: correct error handling without decorator

The decorator hides the number of function arguments from Mercurial,
so Mercurial cannot give proper error messages about commands
invoked with the wrong number of arguments.

Left a 'dummy' hgcommand decorator in place as a way to document
what functions are hg commands, and just in case we need some other
kind of hack in the future.

R=adg, rsc
CC=golang-dev
https://golang.org/cl/6488059