Move "get_ack()" back to fetch-pack

Move "get_ack()" back to fetch-pack

In 41cb7488 Linus moved this function to connect.c for reuse inside
of the git-clone-pack command.  That was 2005, but in 2006 Junio
retired git-clone-pack in commit efc7fa53.  Since then the only
caller has been fetch-pack.  Since this ACK/NAK exchange is only
used by the fetch-pack/upload-pack protocol we should move it back
to be a private detail of fetch-pack.

Signed-off-by: Shawn O. Pearce <redacted>
Signed-off-by: Junio C Hamano <redacted>