fetch-pack: accept "pack" output for packfile URIs

fetch-pack: accept "pack" output for packfile URIs

When index-pack finds an existing keep file it reports pack rather than
keep. Accept either result from http-fetch, and only register a keep
lockfile when this fetch created it.

Read the pack/keep prefix and hash without consuming any following fsck
output, validate the reported pack hash against the advertised hash, and
exercise a packfile URI fetch with a pre-existing keep file.

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