builtin/shortlog: don't set up revisions without repo

builtin/shortlog: don't set up revisions without repo

It is possible to run git-shortlog(1) outside of a repository by passing
it output from git-log(1) via standard input. Obviously, as there is no
repository in that context, it is thus unsupported to pass any revisions
as arguments.

Regardless of that we still end up calling `setup_revisions()`. While
that works alright, it is somewhat strange. Furthermore, this is about
to cause problems when we unset the default object hash.

Refactor the code to only call `setup_revisions()` when we have a
repository. This is safe to do as we already verify that there are no
arguments when running outside of a repository anyway.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>