http-fetch: Use temporary files for pack-*.idx until verified

http-fetch: Use temporary files for pack-*.idx until verified

Verify that a downloaded pack-*.idx file is consistent and valid
as an index file before we rename it into its final destination.
This prevents a corrupt index file from later being treated as a
usable file, confusing readers.

Check that we do not have the pack index file before invoking
fetch_pack_index(); that way, we can do without the has_pack_index()
check in fetch_pack_index().

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>