basic threaded delta search

basic threaded delta search

this is still rough, hence it is disabled by default.  You need to compile
with "make THREADED_DELTA_SEARCH=1 ..." at the moment.

Threading is done on different portions of the object list to be
deltified. This is currently done by spliting the list into n parts and
then a thread is spawned for each of them.  A better method would consist
of spliting the list into more smaller parts and have the n threads
pick the next part available.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>