support pager.* for external commands

support pager.* for external commands

Without this patch, any commands that are not builtin would
not respect pager.* config. For example:

  git config pager.stash false
  git stash list

would still use a pager. With this patch, pager.stash now
has an effect. If it is not specified, we will still fall
back to pager.log when we invoke "log" from "stash list".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>