http-fetch: Use index-pack rather than verify-pack to check packs

To ensure we don't leave a corrupt pack file positioned as though
it were a valid pack file, run index-pack on the temporary pack
before we rename it to its final name.  If index-pack crashes out
when it discovers file corruption (e.g. GitHub's error HTML at the
end of the file), simply delete the temporary files to cleanup.

By waiting until the pack has been validated before we move it
to its final name, we eliminate a race condition where another
concurrent reader might try to access the pack at the same time
that we are still trying to verify its not corrupt.

Switching from verify-pack to index-pack is a change in behavior,
but it should turn out better for users.  The index-pack algorithm
tries to minimize disk seeks, as well as the number of times any
given object is inflated, by organizing its work along delta chains.
The verify-pack logic does not attempt to do this, thrashing the
delta base cache and the filesystem cache.

By recreating the index file locally, we also can automatically
upgrade from a v1 pack table of contents to v2.  This makes the
CRC32 data available for use during later repacks, even if the
server didn't have them on hand.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Acked-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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