config: make `packed_git_(limit|window_size)` non-global variables

config: make `packed_git_(limit|window_size)` non-global variables

The variables `packed_git_window_size` and `packed_git_limit` are global
config variables used in the `packfile.c` file. Since it is only used in
this file, let's change it from being a global config variable to a
local variable for the subsystem.

With this, we rid `packfile.c` from all global variable usage and this
means we can also remove the `USE_THE_REPOSITORY_VARIABLE` guard from
the file.

Helped-by: Taylor Blau <redacted>
Signed-off-by: Karthik Nayak <redacted>
Signed-off-by: Junio C Hamano <redacted>