http: accept HTTP 416 for complete partial packs

http: accept HTTP 416 for complete partial packs

A resumed pack request may already have all bytes of the remote pack.
A server can respond to the resulting Range request with HTTP 416
instead of returning an empty response.

Accept that response in each pack-download caller and let index-pack
validate the completed staging file. This can happen without concurrent
downloads when a previous attempt completed the transfer but failed
before indexing it.

Add a regression test that seeds a complete partial pack and checks that
http-fetch indexes it after the server returns HTTP 416.

Signed-off-by: Ted Nyman <tnyman@openai.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>