ls-refs: ignore very long ref-prefix counts

Because each "ref-prefix" capability from the client comes in its own pkt-line, there's no limit to the number of them that a misbehaving client may send. We read them all into a strvec, which means the client can waste arbitrary amounts of our memory by just sending us "ref-prefix foo" over and over. One possible solution is to just drop the connection when the limit is reached. If we set it high enough, then only misbehaving or malicious clients would hit it. But "high enough" is vague, and it's unfriendly if we guess wrong and a legitimate client hits this. But we can do better. Since supporting the ref-prefix capability is optional anyway, the client has to further cull the response based on their own patterns. So we can simply ignore the patterns once we cross a certain threshold. Note that we have to ignore _all_ patterns, not just the ones past our limit (since otherwise we'd send too little data). The limit here is fairly arbitrary, and probably much higher than anyone would need in practice. It might be worth limiting it further, if only because we check it linearly (so with "m" local refs and "n" patterns, we do "m * n" string comparisons). But if we care about optimizing this, an even better solution may be a more advanced data structure anyway. I didn't bother making the limit configurable, since it's so high and since Git should behave correctly in either case. It wouldn't be too hard to do, but it makes both the code and documentation more complex. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>