http: refactor finish_http_pack_request()

http: refactor finish_http_pack_request()

finish_http_pack_request() does multiple tasks, including some
housekeeping on a struct packed_git - (1) closing its index, (2)
removing it from a list, and (3) installing it. These concerns are
independent of fetching a pack through HTTP: they are there only because
(1) the calling code opens the pack's index before deciding to fetch it,
(2) the calling code maintains a list of packfiles that can be fetched,
and (3) the calling code fetches it in order to make use of its objects
in the same process.

In preparation for a subsequent commit, which adds a feature that does
not need any of this housekeeping, remove (1), (2), and (3) from
finish_http_pack_request(). (2) and (3) are now done by a helper
function, and (1) is the responsibility of the caller (in this patch,
done closer to the point where the pack index is opened).

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>