packfile: use `repository` from `packed_git` directly

packfile: use `repository` from `packed_git` directly

In the previous commit, we introduced the `repository` structure inside
`packed_git`. This provides an alternative route instead of using the
global `the_repository` variable. Let's modify `packfile.c` now to use
this field wherever possible instead of relying on the global state.
There are still a few instances of `the_repository` usage in the file,
where there is no struct `packed_git` locally available, which will be
fixed in the following commits.

Helped-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>