godoc/redirect: update the rules for /cl/ redirect

If id is a number greater than 150k, then it is a Rietveld CL, otherwise
treat it as a Gerrit change id.

Also add support for /cl/ID/, because go commit 0edafefc36 uses this
form.

Change-Id: I46575e3284faaa727e346b34bbc46ab248cf12b3
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/1283
Reviewed-by: Andrew Gerrand <adg@golang.org>