fetch-pack: use a separate flag for fetch in deepening mode

fetch-pack: use a separate flag for fetch in deepening mode

The shallow repo could be deepened or shortened when then user gives
--depth. But in future that won't be the only way to deepen/shorten a
repo. Stop relying on args->depth in this mode. Future deepening
methods can simply set this flag on instead of updating all these if
expressions.

The new name "deepen" was chosen after the command to define shallow
boundary in pack protocol. New commands also follow this tradition.

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