fetch-pack: -k option to keep downloaded pack.

fetch-pack: -k option to keep downloaded pack.

Split out the functions that deal with the socketpair after
finishing git protocol handshake to receive the packed data into
a separate file, and use it in fetch-pack to keep/explode the
received pack data.  We earlier had something like that on
clone-pack side once, but the list discussion resulted in the
decision that it makes sense to always keep the pack for
clone-pack, so unpacking option is not enabled on the clone-pack
side, but we later still could do so easily if we wanted to with
this change.

Signed-off-by: Junio C Hamano <junkio@cox.net>