rev-list: don't show usage when we see empty ref patterns

rev-list: don't show usage when we see empty ref patterns

If the user gives us no starting point for a traversal, we
want to complain with our normal usage message. But if they
tried to do so with "--all" or "--glob", but that happened
not to match any refs, the usage message isn't helpful. We
should just give them the empty output they asked for
instead.

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