object-store: move packed_git and packed_git_mru to object store

In a process with multiple repositories open, packfile accessors
should be associated to a single repository and not shared globally.
Move packed_git and packed_git_mru into the_repository and adjust
callers to reflect this.

[nd: while at there, wrap access to these two fields in get_packed_git()
and get_packed_git_mru(). This allows us to lazily initialize these
fields without caller doing that explicitly]

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

pack-check.c

@@ -3,6 +3,7 @@
 #include "pack-revindex.h"
 #include "progress.h"
 #include "packfile.h"
+#include "object-store.h"
 
 struct idx_entry {
 	off_t                offset;