packfile: allow install_packed_git to handle arbitrary repositories

packfile: allow install_packed_git to handle arbitrary repositories

This conversion was done without the #define trick used in the earlier
series refactoring to have better repository access, because this function
is easy to review, as it only has one caller and all lines but the first
two are converted.

We must not convert 'pack_open_fds' to be a repository specific variable,
as it is used to monitor resource usage of the machine that Git executes
on.

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