cmd/gopherbot: handle backport requests in closed issues
cmd/gopherbot: handle backport requests in closed issues

There is a reasonable use case for going back and backporting a fix for
a closed issue, or the user might have forgotten to open the backport
issues sooner.

Change-Id: I5b9afd10764c7b1438f163ff512ec1e2961b1afa
Reviewed-on: https://go-review.googlesource.com/108915
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build