fetch-pack: return enum from process_acks()

fetch-pack: return enum from process_acks()

process_acks() returns 0, 1, or 2, depending on whether "ready" was
received and if not, whether at least one commit was found to be common.
Replace these magic numbers with a documented enum.

Signed-off-by: Jonathan Tan <redacted>
Reviewed-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>