archive: allow --exec and --remote without equal sign

Allow "--remote repo" and "--exec cmd" in addition to "--remote=repo" and
"--exec=cmd" to make their usage consistent with parameters handled by
parse_options().

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>