dumb-http: do not pass NULL path to parse_pack_index

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>