dumb-http: do not pass NULL path to parse_pack_index

Once upon a time, dumb http always fetched .idx files directly into their final location, and then checked their validity with parse_pack_index. This was refactored in commit 750ef42 (http-fetch: Use temporary files for pack-*.idx until verified, 2010-04-19), which uses the following logic: 1. If we have the idx already in place, see if it's valid (using parse_pack_index). If so, use it. 2. Otherwise, fetch the .idx to a tempfile, check that, and if so move it into place. 3. Either way, fetch the pack itself if necessary. However, it got step 1 wrong. We pass a NULL path parameter to parse_pack_index, so an existing .idx file always looks broken. Worse, we do not treat this broken .idx as an opportunity to re-fetch, but instead return an error, ignoring the pack entirely. This can lead to a dumb-http fetch failing to retrieve the necessary objects. This doesn't come up much in practice, because it must be a packfile that we found out about (and whose .idx we stored) during an earlier dumb-http fetch, but whose packfile we _didn't_ fetch. I.e., we did a partial clone of a repository, didn't need some packfiles, and now a followup fetch needs them. Discovery and tests by Charles Bailey <charles@hashpling.org>. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>