support pager.* for aliases

Until this patch, doing something like:

  git config alias.foo log
  git config pager.foo /some/specific/pager

would not respect pager.foo at all. With this patch, we
will use pager.foo for the "foo" alias.  We will also
fallback to pager.log if "foo" is a non-shell alias that
uses the "log" command (but any pager.foo overrides
pager.log).