upload-pack: change multi_ack to an enum

upload-pack: change multi_ack to an enum

As we cleanup 'upload-pack.c' by using 'struct upload_pack_data'
more thoroughly, let's take this opportunity to change the
'multi_ack' variable, which is now part of 'upload_pack_data',
to an enum.

This will make it clear which values this variable can take.

Helped-by: Jonathan Tan <redacted>
Signed-off-by: Christian Couder <redacted>
Acked-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>