Small fix to writing Gnus dribble change-level entries

* lisp/gnus/gnus-start.el (gnus-group-change-level): PREVIOUS needs to
  still be a string when the dribble entry is written, so don't
  convert it to an entry until after that's done. Also, we're not
  meant to write PREVIOUS itself, we're meant to write the group that
  comes _after_ it in the sort-order of gnus-group-list, so do that
  instead.
1 job for master in 51 minutes and 32 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
failed #2362
test-all

00:51:32

 
Name Stage Failure
failed
test-all Test
make[2]: *** [check-doit] Error 1
Makefile:318: recipe for target 'check-doit' failed
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:293: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
make: *** [check-expensive] Error 2
Makefile:959: recipe for target 'check-expensive' failed
ERROR: Job failed: exit code 1