Merge branch 'jk/pager-bypass-cat-for-default-pager'

If a build-time fallback is set to "cat" instead of "less", we
should apply the same "no subprocess or pipe" optimization as we
apply to user-supplied GIT_PAGER=cat.

* jk/pager-bypass-cat-for-default-pager:
  pager: turn on "cat" optimization for DEFAULT_PAGER