Make gnus-newsgroup-name local var available to gnus-summary-mode

* lisp/gnus/gnus-sum.el (gnus-summary-mode-group): New defvar for
  passing gnus-newsgroup-name (a local variable) into the body of
  gnus-summary-mode, so that it's available to gnus-summary-mode-hook.
1 job for master in 3 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #340
test

00:00:03

 
Name Stage Failure
failed
test Test There has been a runner system failure, please try again
Running with gitlab-runner 11.4.2 (cf91d5e1)
on gitlab-runner-docker cff9d373
Using Docker executor with image debian:stretch ...
Pulling docker image debian:stretch ...
Using docker image sha256:4879790bd60d439cfe39c063660eef7af525d5f6f1cbb701a14c7cfc11cbfcf7 for debian:stretch ...
ERROR: Job failed (system failure): Error response from daemon: open /var/run/docker/libcontainerd/containerd/1b3f495175cb6b76c67820a5ca80130d5de1a49fe372627499dd29032dbf3a05/init/shim-log.json: no such file or directory (executor_docker.go:1015:0s)