A common error is to do "git-p4 sync" in a repository that
was not initialized by "git-p4 clone". There will be no
p4 refs. The error message in this case is a traceback
for an assertion, which is confusing.
Change it instead to explain the likely problem.
Signed-off-by: Pete Wyckoff <pw@padd.com> Acked-By: Tor Arvid Lund <torarvid@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>