main: fix pager logic after negation cleanup

The pager setting is tristate (where None means "auto"),
so make sure we still handle that setting.

Change-Id: I89fe352572dd15922c61e3bb65ac33f847d01ee0
Test: `repo help upload` triggers the pager
Test: `repo -p help upload` triggers the pager
Test: `repo --no-pager help upload` doesn't trigger the pager
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/255852


Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>