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 <bradfitz@golang.org>
Reviewed-by: Chris Broadfoot <cbro@golang.org>
1 file changed