(repos-count-screen-lines): Narrow without changing point-min

* lisp/reposition.el (repos-count-screen-lines): Narrow without
changing point-min.  It wasn't necessary for vertical-motion to
work, and it had some adverse effects on how font-lock rules were
applied
(https://lists.gnu.org/archive/html/emacs-devel/2019-11/msg00522.html).
This can also be an alternative fix for bug#38049.
1 job for master in 65 minutes and 12 seconds (queued for 7 seconds)
Status Job ID Name Coverage
  Test
failed #4224
test-all

01:05:12

 
Name Stage Failure
failed
test-all Test
Makefile:319: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:294: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:959: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1