cmd/pushback: use friendlier message when closing pull requests
cmd/pushback: use friendlier message when closing pull requests

Fixes golang/go#13687

Change-Id: I87eb041b97c7368b40c1f578e9f71313570cf7f6
Reviewed-on: https://go-review.googlesource.com/19116
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Chris Broadfoot <0c2851a4f8ab4aa5606e69c0757771dd13dbe686@golang.org>

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