http: use --stdin when indexing dumb HTTP pack

http: use --stdin when indexing dumb HTTP pack

When Git fetches a pack using dumb HTTP, (among other things) it invokes
index-pack on a ".pack.temp" packfile, specifying the filename as an
argument.

A future commit will require the aforementioned invocation of index-pack
to also generate a "keep" file. To use this, we either have to use
index-pack's naming convention (because --keep requires the pack's
filename to end with ".pack") or to pass the pack through stdin. Of the
two, it is simpler to pass the pack through stdin.

Thus, teach http to pass --stdin to index-pack. As a bonus, the code is
now simpler.

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