diff: fix crash with --find-object outside repository

diff: fix crash with --find-object outside repository

When "git diff --find-object=<oid>" is run outside a git repository,
the option parsing callback eagerly resolves the OID via
repo_get_oid(), which reaches get_main_ref_store() and hits a BUG()
assertion because no repository has been set up.

Check startup_info->have_repository before attempting to resolve the
OID, and return a user-friendly error instead.

Signed-off-by: Michael Montalbo <mmontalbo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>