fetch-pack: use a common function for verbose printing

This reduces the number of "if (verbose)" which makes it a bit easier
to read imo. It also makes it easier to redirect all these printouts,
to a file for example.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>