git-stash: don't complain when listing in a repo with no stash

git-stash: don't complain when listing in a repo with no stash

Previously, the git-log invocation would complain if a repo
had not had any stashes created in it yet:

$ git-init
$ git-stash
fatal: ambiguous argument 'refs/stash': unknown revision or
  path not in the working tree.
Use '--' to separate paths from revisions

Instead, we only call git-log if we actually have a
refs/stash. We could alternatively create the ref when any
stash command is called, but it's better for the 'list'
command to not require write access to the repo.

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