archive: don't let remote clients get unreachable commits

Usually git is careful not to allow clients to fetch
arbitrary objects from the database; for example, objects
received via upload-pack must be reachable from a ref.
Upload-archive breaks this by feeding the client's tree-ish
directly to get_sha1, which will accept arbitrary hex sha1s,
reflogs, etc.

This is not a problem if all of your objects are publicly
reachable anyway (or at least public to anybody who can run
upload-archive). Or if you are making the repo available by
dumb protocols like http or rsync (in which case the client
can read your whole object db directly).

But for sites which allow access only through smart
protocols, clients may be able to fetch trees from commits
that exist in the server's object database but are not
referenced (e.g., because history was rewound).

This patch tightens upload-archive's lookup to use dwim_ref
rather than get_sha1. This means a remote client can only
fetch the tip of a named ref, not an arbitrary sha1 or
reflog entry.

This also restricts some legitimate requests, too:

  1. Reachable non-tip commits, like:

        git archive --remote=$url v1.0~5

  2. Sub-trees of reachable commits, like:

        git archive --remote=$url v1.7.7:Documentation

Local requests continue to use get_sha1, and are not
restricted at all.

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

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