upload-pack: use a strmap for want-ref lines

When the "ref-in-want" capability is advertised (which it is not by default), then upload-pack processes a "want-ref" line from the client by checking that the name is a valid ref and recording it in a string-list. In theory this list should grow no larger than the number of refs in the server-side repository. But since we don't do any de-duplication, a client which sends "want-ref refs/heads/foo" over and over will cause the array to grow without bound. We can fix this by switching to strmap, which efficiently detects duplicates. There are two client-visible changes here: 1. The "wanted-refs" response will now be in an apparently-random order (based on iterating the hashmap) rather than the order given by the client. The protocol documentation is quiet on ordering here. The current fetch-pack implementation is happy with any order, as it looks up each returned ref using a binary search in its local sorted list. JGit seems to implement want-ref on the server side, but has no client-side support. libgit2 doesn't support either side. It would obviously be possible to record the original order or to use the strmap as an auxiliary data structure. But if the client doesn't care, we may as well do the simplest thing. 2. We'll now reject duplicates explicitly as a protocol error. The client should never send them (and our current implementation, even when asked to "git fetch master:one master:two" will de-dup on the client side). If we wanted to be more forgiving, we could perhaps just throw away the duplicates. But then our "wanted-refs" response back to the client would omit the duplicates, and it's hard to say what a client that accidentally sent a duplicate would do with that. So I think we're better off to complain loudly before anybody accidentally writes such a client. Let's also add a note to the protocol documentation clarifying that duplicates are forbidden. As discussed above, this was already the intent, but it's not very explicit. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>